Currently, the compositor can be disabled using the WITH_COMPOSITOR
build option. Since, we intent to always build the realtime compositor,
we need to make the distinction between both compositors clear.
So this patch renames the option to WITH_COMPOSITOR_CPU. Additionally,
the check for the option was moved inside the compositor modules' own
CMake file in preparation for the realtime compositor code.
Differential Revision: https://developer.blender.org/D15622
Reviewed By: Jeroen Bakker, Ray Molenkamp
9990792e87