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

src/server/shared/Debugging/WheatyExceptionReport.cpp

@@ -3,7 +3,7 @@
 // MSDN Magazine, 2002
 // FILE: WheatyExceptionReport.CPP
 //==========================================
-#if PLATFORM == PLATFORM_WINDOWS
+#if PLATFORM == PLATFORM_WINDOWS && not defined(__MINGW32__)
 #define WIN32_LEAN_AND_MEAN
 #pragma warning(disable:4996)
 #pragma warning(disable:4312)

src/server/shared/Debugging/WheatyExceptionReport.h

@@ -1,7 +1,7 @@
 #ifndef _WHEATYEXCEPTIONREPORT_
 #define _WHEATYEXCEPTIONREPORT_
 
-#if PLATFORM == PLATFORM_WINDOWS
+#if PLATFORM == PLATFORM_WINDOWS && not defined(__MINGW32__)
 
 #include <dbghelp.h>