Move wxCHECK_MINGW32_VERSION() declaration to wx/msw/gccpriv.h.

Unfortunately we can't keep it in wx/compiler.h which is included too early,
before UNICODE macro is defined, and we can't define it before including
wx/setup.h which itself must be included after wx/compiler.h.

So move it to wx/msw/gccpriv.h which is not a great solution, as the best
would be to get rid of this gcc-specific file completely, but at least should
make everything build again and ensure that _mingw.h is included after UNICODE
definition.

Closes #15805.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75555 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/platform.h

@@ -456,6 +456,10 @@
 #define wxNEEDS_CHARPP
 #endif
 
+/*
+    Note that wx/msw/gccpriv.h must be included after defining UNICODE and
+    _UNICODE macros as it includes _mingw.h which relies on them being set.
+ */
 #if ( defined( __GNUWIN32__ ) || defined( __MINGW32__ ) || \
     ( defined( __CYGWIN__ ) && defined( __WINDOWS__ ) ) || \
       wxCHECK_WATCOM_VERSION(1,0) ) && \
@@ -467,6 +471,8 @@
 #else
 #    undef wxCHECK_W32API_VERSION
 #    define wxCHECK_W32API_VERSION(maj, min) (0)
+#    undef wxCHECK_MINGW32_VERSION
+#    define wxCHECK_MINGW32_VERSION( major, minor ) (0)
 #endif