compiler-versions.cmake: refine versions requirements.
compiler-versions.cmake: refine versions requirements.

* Accept clang-3.5 if OpenMP support is not required.
  (if OpenMP support is required, still only accept clang-3.9)
* Add XCode version checks.
   With OpenMP - 10.0 (clang-6.0svn?)
   Without OpenMP - 6.0 (based on clang-3.5svn)

darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed