Fix raygui build on windows, and indicate build commands in readme

[ToreAad]

Hi!

I had some issues building the prebuild branch on windows. The two changes indicated in the PR fixed it for me:

1. I build on windows using msys2 gcc and had to tweak the command that built the binaries slightly.
2. I had some trouble building raygui due to some name collisions with windows stuff.

I don't know how much my experience relates to others trying to build on windows, but maybe they would find these changes helpful.

Thank you for maintaining claw-raylib.

Tore

[bohonghuang]

Hello, thank you for your efforts! Could you please change the target of the PR to master? The prebuild branch is used for providing precompiled adapters and will be force-pushed to stay one commit ahead of master periodically.