Remove obsolete defines from the buildsystem

[JohannesKauffmann]

None of these defines are used anywhere in the code, so they should be safe to delete.

[JohannesKauffmann]

Thanks for the review, I indeed didn't know those defines were used that way.

Would it be okay to scope those defines to macOS and Emscripten? The macOS one is used unconditionally, and it seems the Emscripten define is only used for Windows builds?

[kblaschke]

Huh, right, now I see. So then just remove USE_FTGL.

For GL_SILENCE_DEPRECATION, use a generator expression to limit it to Darwin:

add_compile_definitions(
        DATADIR_PATH="${PROJECTM_DATADIR_PATH}"
        $<IF:$<PLATFORM_ID:Darwin>,GL_SILENCE_DEPRECATION>
        )

[JohannesKauffmann]

Hi, sorry for getting back so late on this.

I've rebased on master, and also included a minor refactor with the Windows-only defines switched to generator expressions.