Add support for Cocoa OpenGL screen in gfxlib2 and other compiler error fixes for macOS

[Markos-Th09]

This PR includes several changes to improve support for the macOS platform in fbc and its libraries. It also aims to add support for OpenGL screens using Cocoa instead of requiring XQuartz for X11 support, although at the time of writing this I haven't tested the implementation very thoroughly yet.

Currently some mechanism similar to #inclib but for macOS frameworks is missing. Since a lot of important system libraries contained within frameworks such as OpenGL, this would be useful for headers such as gl.bi and glut.bi but also in general. How should this feature be implemented in FreeBASIC?