Add MSYS/MinGW compiling support on Windows

Tested successfully with MSYS2 with the following command:
scons platform=windows generate_bindings=yes use_mingw=1 bits=64

SConstruct

@@ -16,7 +16,7 @@ if sys.platform.startswith('linux'):
     host_platform = 'linux'
 elif sys.platform == 'darwin':
     host_platform = 'osx'
-elif sys.platform == 'win32':
+elif sys.platform == 'win32' or sys.platform == 'msys':
     host_platform = 'windows'
 else:
     raise ValueError(