emu-compatibility.com is now defunct and thus should not be in About menu.
Other minor changes:
* linkage-rwx-linux-elf.diff -> linkage-rwx-linux-elf.patch
* Mark some inputs as optional
* Do not build with Doxygen by default: it does not produce any outputs
* Do not build with OpenAL by default: SDL2 handles sound when present
* Do not build with FreeGLUT by default: deprecated at upstream
