Fix building and using the library with MinGW -std=c++{98,11} options.

These options enable "strict ANSI" mode in MinGW which omits declarations of
POSIX functions from the standard headers. To allow the library and, possibly
even more importantly, the user code including our headers, to compile with
these options, declare the functions that we need ourselves.

This might appear to go against the spirit of "strict ANSI" mode, but the only
alternative would be to not use such functions at all and silently cripple the
library when -std=c++NN is used, compared to -std=g++NN case, and this doesn't
seem appealing neither.

Closes #16984.

include/wx/msw/gccpriv.h

@@ -132,5 +132,28 @@
     #endif
 #endif
 
+/*
+    Traditional MinGW (but not MinGW-w64 nor TDM-GCC) omits many POSIX
+    functions from their headers when compiled with __STRICT_ANSI__ defined.
+    Unfortunately this means that they are not available when using -std=c++98
+    (not very common) or -std=c++11 (much more so), but we still need them even
+    in this case. As the intention behind using -std=c++11 is probably to get
+    the new C++11 features and not disable the use of POSIX functions, we just
+    manually declare the functions we need in this case if necessary.
+ */
+#if defined(__MINGW32_TOOLCHAIN__) && defined(__STRICT_ANSI__)
+    #define wxNEEDS_STRICT_ANSI_WORKAROUNDS
+
+    /*
+        This macro is somewhat unusual as it takes the list of parameters
+        inside parentheses and includes semicolon inside it as putting the
+        semicolon outside wouldn't do the right thing when this macro is empty.
+     */
+    #define wxDECL_FOR_STRICT_MINGW32(rettype, func, params) \
+        extern "C" _CRTIMP rettype __cdecl __MINGW_NOTHROW func params ;
+#else
+    #define wxDECL_FOR_STRICT_MINGW32(rettype, func, params)
+#endif
+
 #endif
   /* _WX_MSW_GCCPRIV_H_ */