Fix CPU detection and SIMD on modern Mac/cmake combos

Fix CPU detection and SIMD on modern Mac/cmake combos

With recent versions of cmake on Mac, the CPU detection
program failed to compile since it was very fragile (buggy)
with respect to how compiler flags were defined - and that
in turn will mean losing all SIMD support.
This will fix the build, and it also adds a more vocal warning
if the CPU detection code cannot be compiled at all.

Change-Id: Idcdc8b59a4252b3faf8572311d2ca63a4d2a1bc0