Basically title says it all.
The goal is to make platform maintenance easier, so you don't have
to constantly scroll back and forth looking for if() branches to
check which exact platform you're currently working on.
Ideally we also would move option defaults to a platform files,
but that i'm not sure how to implement in a nice way yet.
Reviewers: mont29
Reviewed By: mont29
Differential Revision: https://developer.blender.org/D2148
