OPTIONS.md: Fix subscripts for USE_OPEN{G,A}L
CMakeLists.txt: Merge USE_SDL1 and USE_SDL2

Only one of both can be used at the same time so it does not make sense
to toggle them independently. Instead, add a WITH_SDL config variable
to allow selecting a specific version if desired.
CMakeLists.txt: Merge USE_QT4 and USE_QT5

Only one of both can be used at the same time so it does not make sense
to toggle them independently. Instead, add a WITH_QT config variable
to allow selecting a specific version if desired.
CMakeLists.txt: Cleanup ARX_FOUND_QT_VERSION logic