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/filefn.h

@@ -216,8 +216,22 @@ enum wxPosixPermissions
         #define wxFtell _ftelli64
     #elif wxCHECK_MINGW32_VERSION(3, 5) // mingw-runtime version (not gcc)
         #define wxHAS_HUGE_STDIO_FILES
+
+        wxDECL_FOR_STRICT_MINGW32(int, fseeko64, (FILE*, long long, int));
         #define wxFseek fseeko64
-        #define wxFtell ftello64
+
+        #ifdef wxNEEDS_STRICT_ANSI_WORKAROUNDS
+            // Unfortunately ftello64() is not defined in the library for
+            // whatever reason but as an inline function, so define wxFtell()
+            // here similarly.
+            inline long long wxFtell(FILE* fp)
+            {
+                fpos_t pos;
+                return fgetpos(fp, &pos) == 0 ? pos : -1LL;
+            }
+        #else
+            #define wxFtell ftello64
+        #endif
     #endif
 
     // other Windows compilers (DMC, Watcom, and Borland) don't have huge file
@@ -376,7 +390,7 @@ enum wxPosixPermissions
 
     // finally the default char-type versions
     #if wxUSE_UNICODE
-        #if wxUSE_UNICODE_MSLU || defined(__WX_STRICT_ANSI_GCC__)
+        #if wxUSE_UNICODE_MSLU
             // implement the missing file functions in Win9x ourselves
             WXDLLIMPEXP_BASE int wxMSLU__wopen(const wxChar *name,
                                                int flags, int mode);
@@ -404,6 +418,9 @@ enum wxPosixPermissions
             #define wxCRT_MkDir     wxCRT_MkDirW
             #define wxCRT_RmDir     wxCRT_RmDirW
             #define wxCRT_Stat      wxCRT_StatW
+
+            wxDECL_FOR_STRICT_MINGW32(int, _wmkdir, (const wchar_t*))
+            wxDECL_FOR_STRICT_MINGW32(int, _wrmdir, (const wchar_t*))
         #endif // wxUSE_UNICODE_MSLU/!wxUSE_UNICODE_MSLU
     #else // !wxUSE_UNICODE
         #define wxCRT_Open      wxCRT_OpenA