Make raygui.zig nicer

[AndrewKraevskii]

1. Return void if raylib function always returns value of 0.
2. Return bool if raylib returns 1 of 0.
3. Use enum for guiSetState and guiGetState.
4. Convert i32 to ?u32 if it can be -1 or positive.
5. Also link libc for tests because they can't be run overwise.

further improvements: use u32 instead of i32 for guiSetStyle and gutGetStyle as it can only be positive in raygui code (used for array access and array is unsigned char) but im not sure about that

[Not-Nik]

I'm hesitant to merge this, at the very least it needs a lot of work.

• I strongly disagree with some of the formatting in your build.zig. Also the test targets aren't intended to be run, they are there to check for syntax errors in the binding files, so all of your changes to the build file are unnecessary. If there were changes in need to be made, they should be in a separate PR.
• All the changes you made to raygui.zig should be integrated with generate_functions.py. There already is code for replacing enums, and for the void/bool I would suggest an extra array each (FORCE_RETURN_VOID, FORCE_RETURN_BOOL) to change the return type in the cdef. This way we don't need to cast any of the return values.
• Instead of returning ?u32 I'd rather have functions return ?i32.
• The suggested changes for guiSetStyle and guiGetStyle would probably cause UB and also wouldn't be handled properly on the raygui side for values > 2^31.