Buildsystem: Add support for compiling with MinGW on Windows

Tested with:
- Windows 8 x64
- MySQL 5.5.30 win32
- OpenSSL 1.0.1c (32 bits)
- No PCH
- MinGW with GCC 4.7.0

TODO:
- Fix compile/link with PCH enabled
- Fix compile with WheatyExceptonionReport enabled (ignored for now)
- Fix compile of .rc files (ignored for now)
- Test with more platforms

dep/g3dlite/source/FileSystem.cpp

@@ -25,8 +25,11 @@
 
     // Needed for _findfirst
 #   include <io.h>
-
-#define stat64 _stat64
+#  ifdef __MINGW32__
+#    define stat64 stat
+#  else
+#    define stat64 _stat64
+#  endif
 #else
 #   include <dirent.h>
 #   include <fnmatch.h>