Blender Git Loki

Cycles: Reduce scope of some defines set in CMakeLists

Should be no functional changes at all, just speeds up re-compilation
when some features needs to be disabled for development purposes.

For example, when running lots of Valgrind it's handy to disable any
GPU devices because otherwise you'll be wasting quite some time in
the driver while enumerating devices.

Reviewers: dingto, lukasstockner97, brecht, juicyfruit

Differential Revision: https://developer.blender.org/D1730