The C89 requirement is a wall that is hit by many software projects (PROJ.4, GDAL, MapServer/MapCache?) as many different compilers support different standards.

MapServer/MapCache? recently added the -Werror=declaration-after-statement to avoid compiler errors when variables are not declared at the beginning of the block: ​https://github.com/mapserver/mapcache/commit/219e54b45e4730b91111a6874e435a8ca8a1aa6d

Even Rouault recently mentioned that PROJ.4 also uses that flag, which is based on the GDAL use of it also: ​https://github.com/OSGeo/proj.4/blob/master/m4/ax_cflags_warn_all.m4#L35

Here are more ZOO-Project compile errors on Windows:

d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(273) : error C2061: syntax error : identifier 'hasKey'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(273) : error C2059: syntax error : ';'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(273) : error C2059: syntax error : 'type'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(285) : error C2061: syntax error : identifier 'hasElement'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(285) : error C2059: syntax error : ';'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(285) : error C2059: syntax error : 'type'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(297) : error C2061: syntax error : identifier 'contains'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(297) : error C2059: syntax error : ';'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(297) : error C2059: syntax error : 'type'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(316) : error C2061: syntax error : identifier 'addServiceToRegistry'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(316) : error C2059: syntax error : ';'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(316) : error C2059: syntax error : 'type'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(328) : warning C4273: '_snprintf' : inconsistent dll linkage
        C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\INCLUDE\stdio.h(349) : see previous definition of '_snprintf'
ulinet.c(92) : error C2143: syntax error : missing ';' before 'type'
ulinet.c(93) : error C2275: '_HINTERNET' : illegal use of this type as an expression
        d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\ulinet.h(87) : see declaration of '_HINTERNET'
ulinet.c(93) : error C2065: 'psInternet' : undeclared identifier
ulinet.c(94) : error C2065: 'psInternet' : undeclared identifier
ulinet.c(94) : warning C4047: '!=' : 'int' differs in levels of indirection from 'void *'
ulinet.c(95) : error C2065: 'psInternet' : undeclared identifier
ulinet.c(95) : error C2223: left of '->cookie' must point to struct/union
ulinet.c(96) : error C2065: 'psInternet' : undeclared identifier
ulinet.c(96) : error C2223: left of '->cookie' must point to struct/union
ulinet.c(246) : warning C4013: 'strtok_r' undefined; assuming extern returning int
ulinet.c(246) : warning C4047: '=' : 'char *' differs in levels of indirection from 'int'
ulinet.c(247) : error C2143: syntax error : missing ';' before 'type'
ulinet.c(248) : error C2143: syntax error : missing ';' before 'type'
ulinet.c(249) : error C2065: 'cnt' : undeclared identifier
ulinet.c(251) : error C2065: 'cnt' : undeclared identifier
ulinet.c(254) : error C2065: 'cnt' : undeclared identifier
ulinet.c(258) : warning C4047: '=' : 'char *' differs in levels of indirection from 'int'
ulinet.c(259) : error C2065: 'cnt' : undeclared identifier
ulinet.c(281) : warning C4047: '=' : 'char *' differs in levels of indirection from 'int'
ulinet.c(282) : error C2143: syntax error : missing ';' before 'type'
ulinet.c(283) : error C2065: 'i' : undeclared identifier
ulinet.c(284) : error C2065: 'i' : undeclared identifier
ulinet.c(289) : error C2143: syntax error : missing ';' before 'type'
ulinet.c(290) : error C2065: 'j' : undeclared identifier
ulinet.c(291) : error C2065: 'j' : undeclared identifier
ulinet.c(292) : error C2065: 'j' : undeclared identifier
ulinet.c(292) : error C2065: 'i' : undeclared identifier
ulinet.c(294) : error C2065: 'j' : undeclared identifier
ulinet.c(295) : error C2065: 'j' : undeclared identifier
ulinet.c(298) : error C2275: 'map' : illegal use of this type as an expression
        d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(191) : see declaration of 'map'
ulinet.c(298) : error C2065: 'tmpMap' : undeclared identifier
ulinet.c(299) : error C2065: 'tmpMap' : undeclared identifier
ulinet.c(299) : warning C4047: '!=' : 'int' differs in levels of indirection from 'void *'
ulinet.c(300) : error C2065: 'i' : undeclared identifier
ulinet.c(300) : error C2065: 'tmpMap' : undeclared identifier
ulinet.c(300) : error C2223: left of '->value' must point to struct/union
ulinet.c(300) : error C2198: 'AddMissingHeaderEntry' : too few arguments for call
ulinet.c(302) : error C2065: 'i' : undeclared identifier
ulinet.c(303) : error C2065: 'i' : undeclared identifier
ulinet.c(305) : warning C4047: '=' : 'char *' differs in levels of indirection from 'int'
ulinet.c(539) : warning C4018: '<' : signed/unsigned mismatch
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\BIN\cl.EXE"' : return code '0x2'
Stop.

• unistd.h is not part of Visual Studio
• add a check for that in zoo-kernel/service_internal.h, around line#80, such as:

    #if !defined(USE_RUBY) && !defined(WIN32)
    #include <unistd.h>
    #endif
  

• using prototype-v0 branch
• compile errors with zoo-kernel:

  service_internal_mono.c
  d:\build\mapserver-buildkit-2017\zoo-project-prototype-v0\zoo-kernel\service.h(66): warning C4244: '=': conversion from '__time64_t' to 'long', possible loss of data
  service_internal_mono.c(176): error C2664: 'maps *mapsFromZMaps(MonoDomain *,MonoClass **,MonoMethod **,MonoObject *)': cannot convert argument 4 from 'void *' to 'MonoObject *'
  service_internal_mono.c(176): note: Conversion from 'void*' to pointer to non-'void' requires an explicit cast
  service_internal_mono.c(181): error C2664: 'maps *mapsFromZMaps(MonoDomain *,MonoClass **,MonoMethod **,MonoObject *)': cannot convert argument 4 from 'void *' to 'MonoObject *'
  service_internal_mono.c(181): note: Conversion from 'void*' to pointer to non-'void' requires an explicit cast
  service_internal_mono.c(290): error C2440: 'initializing': cannot convert from 'MonoString *' to 'MonoObject *'
  service_internal_mono.c(290): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
  service_internal_mono.c(291): error C2664: 'char *mono_string_to_utf8(MonoString *)': cannot convert argument 1 from 'MonoObject *' to 'MonoString *'
  service_internal_mono.c(291): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
  service_internal_mono.c(294): error C2440: 'initializing': cannot convert from 'MonoString *' to 'MonoObject *'
  service_internal_mono.c(294): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
  service_internal_mono.c(331): error C2440: 'initializing': cannot convert from 'MonoString *' to 'MonoObject *'
  service_internal_mono.c(331): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
  service_internal_mono.c(332): error C2664: 'char *mono_string_to_utf8(MonoString *)': cannot convert argument 1 from 'MonoObject *' to 'MonoString *'
  service_internal_mono.c(332): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
  service_internal_mono.c(335): error C2440: 'initializing': cannot convert from 'MonoString *' to 'MonoObject *'
  service_internal_mono.c(335): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
  service_internal_mono.c(336): error C2664: 'char *mono_string_to_utf8(MonoString *)': cannot convert argument 1 from 'MonoObject *' to 'MonoString *'
  service_internal_mono.c(336): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
  service_internal_mono.c(352): error C2664: 'char *mono_string_to_utf8(MonoString *)': cannot convert argument 1 from 'const char *' to 'MonoString *'
  service_internal_mono.c(352): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
  service_internal_mono.c(352): error C2660: 'mono_string_new': function does not take 1 arguments
  D:\build\mapserver-buildkit-2017\zoo-project-prototype-v0\..\mono-5.18.0.240/msvc/include\mono/metadata/object.h(146): note: see declaration of 'mono_string_new'
  service_internal_mono.c(366): error C2440: 'initializing': cannot convert from 'int *' to 'int'
  service_internal_mono.c(366): note: There is no context in which this conversion is possible
  service_internal_mono.c(367): error C2446: '>': no conversion from 'int' to 'int *'
  service_internal_mono.c(367): note: Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
  NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\bin\HostX86\x86\cl.EXE"' : return code '0x2'
  Stop.