Apple's desktop operating system was formerly called "Mac OS X" and "OS X", but since 2016 it has been called "macOS" (starting with version 10.12).
Changing across all comments and documentation to use this current terminology, except in cases where the historical versions are explicitly referenced. No code changes are made; we should consider changing those in future PRs.
With GLAD it's not needed anymore to link against OpenGL libraries.
As configure knows better whether a platform supports GLAD, detection is
moved here.
To allow platforms where OpenGL should be linked at build time, the
tests are kept. Android uses them for now.
The OpenGL for Games was completely decoupled from the OpenGL for 2D
engines. This not the case anymore.
