Add Windows Build Script

Build Raylib from source (master branch) and put `raylib.lib` somewhere
on the path. Then run with `python build_win32.py` to build the
extension.

A prebuilt extension is provided for Win32 x64 Python37.

raylib/build_win32.py

@@ -0,0 +1,17 @@
+# Windows build assumes raylib, OpenGL32, etc are all already installed as system libraries.  We dont distribute them.
+
+from cffi import FFI
+
+ffibuilder = FFI()
+ffibuilder.cdef(open("raylib_modified.h").read().replace('RLAPI ', ''))
+ffibuilder.set_source("_raylib_cffi",
+	"""
+	#include "raylib.h"   // the C header of the library
+	""",
+	extra_link_args=['/NODEFAULTLIB:MSVCRTD'],
+	libraries=['raylib', 'gdi32', 'shell32', 'user32','OpenGL32', 'winmm'],
+)
+
+
+if __name__ == "__main__":
+    ffibuilder.compile(verbose=True)