fix platform detection for Windows when using MSYS2

wscript

@@ -21,7 +21,7 @@ def build(bld):
     includes += ["libs/glcxx/include"]
     if platform.system() == "Linux":
         defines += ["PLATFORM_LINUX"]
-    elif platform.system() == "Windows":
+    elif re.search(r'MINGW', platform.system()):
         defines += ["PLATFORM_WINDOWS"]
     defines += ['GLCXX_GL_INCLUDE="gl3w.h"']
     sources = bld.path.ant_glob(["src/**/*.cc", "src/**/*.c", "libs/glcxx/src/glcxx/*"])