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

cmake/platform/win/settings.cmake

@@ -25,4 +25,8 @@ endif()
 #  endif()
 #endif()
 
-include(${CMAKE_SOURCE_DIR}/cmake/compiler/msvc/settings.cmake)
+if ( MSVC )
+  include(${CMAKE_SOURCE_DIR}/cmake/compiler/msvc/settings.cmake)
+elseif ( MINGW )
+  include(${CMAKE_SOURCE_DIR}/cmake/compiler/mingw/settings.cmake)
+endif()