Move __MINGW64_TOOLCHAIN__ definition to wx/msw/gccpriv.h.

__MINGW64_VERSION_MAJOR is defined only in _mingw.h, so it was never defined
in wx/compiler.h, included before any other headers and we can't include
_mingw.h for the same reason we must define wxCHECK_MINGW32_VERSION() in
wx/msw/gccpriv.h: this has to be done after defining UNICODE/_UNICODE, but
wx/compiler.h is included before this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77532 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/gccpriv.h

@@ -35,6 +35,28 @@
 
     #include <_mingw.h>
 
+    /*
+        MinGW-w64 project provides compilers for both Win32 and Win64 but only
+        defines the same __MINGW32__ symbol for the former as MinGW32 toolchain
+        which is quite different (notably doesn't provide many SDK headers that
+        MinGW-w64 does include). So we define a separate symbol which, unlike
+        the predefined __MINGW64__, can be used to detect this toolchain in
+        both 32 and 64 bit builds.
+
+        And define __MINGW32_TOOLCHAIN__ for consistency and also because it's
+        convenient as we often want to have some workarounds only for the (old)
+        MinGW32 but not (newer) MinGW-w64, which still predefines __MINGW32__.
+     */
+    #ifdef __MINGW64_VERSION_MAJOR
+        #ifndef __MINGW64_TOOLCHAIN__
+            #define __MINGW64_TOOLCHAIN__
+        #endif
+    #else
+        #ifndef __MINGW32_TOOLCHAIN__
+            #define __MINGW32_TOOLCHAIN__
+        #endif
+    #endif
+
     #define wxCHECK_MINGW32_VERSION( major, minor ) \
  ( ( ( __MINGW32_MAJOR_VERSION > (major) ) \
       || ( __MINGW32_MAJOR_VERSION == (major) && __MINGW32_MINOR_VERSION >= (minor) ) ) )