There is not much sense to have a whole BLI file just to check SSE2 on CPUs...
So idea is to rename it to more generic "BLI_system", and add to it more system-related
utils, like e.g. an include helper for getpid(), which allows to hide unix/windows
internals from rest of the code...
Reviewers: campbellbarton
Reviewed By: campbellbarton
Differential Revision: https://developer.blender.org/D439
by default now on all platforms since it shouldn't crash anymore if SSE is
not there. If this breaks compilation on some platforms, please let me know.