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.

Using Visual Studio 2012, the following errors are thrown:

ulinet.c
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(58) : error C2143: syntax error : missing ';' before 'type'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(59) : error C2065: 'theTime' : undeclared identifier
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(59) : warning C4133: 'function' : incompatible types - from 'int *' to '__timeb64 *'
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(60) : error C2065: 'theTime' : undeclared identifier
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(60) : error C2224: left of '.time' must have struct/union type
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(61) : error C2065: 'theTime' : undeclared identifier
d:\build\mapserver-buildkit-2012\zoo-project-svn-20170731\zoo-project\zoo-kernel\service.h(61) : error C2224: left of '.millitm' must have struct/union type

This is due to the C89-style requirement, where variables must be declared at beginning of scope.

Using Visual Studio 2012 compiler, the following errors are thrown:

request_parser.c(1743) : error C2440: 'initializing' : cannot convert from 'const char *' to 'char *'
        Conversion loses qualifiers
request_parser.c(1792) : error C2440: 'initializing' : cannot convert from 'const char *' to 'char *'
        Conversion loses qualifiers
request_parser.c(1795) : error C2440: 'initializing' : cannot convert from 'const char *' to 'char *'
        Conversion loses qualifiers
request_parser.c(1807) : error C2440: '=' : cannot convert from 'const char *' to 'char *'
        Conversion loses qualifiers
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\BIN\cl.EXE"' : return code '0x2'
Stop.

more about this error: ​https://msdn.microsoft.com/en-us/library/sy5tsf8z.aspx

I am attaching a possible fix for this (please review if it is valid).