disable OpenMP with all OS X gcc 4.2-based compilers

Although gcc 4.2 should have OpenMP support, the gcc 4.2.1-based
compilers on Mac OS X (defaults in 10.6.x) all claim to support OpenMP,
but generates segfaulting code.

This change reworks the llvm-specific check and moves it out from the
C/CXX flag generation module.

This compiler is the default on many BSD os-es, but as no other gcc
4.2.x has been tested yet (to my knowledge), for now the limitation is
introduced only for the Mac OS X and gcc 4.2.x.

Refs #900

Change-Id: I1c2a27f6fc1162cf8999c65ff6173121109cfbad
Merge "disable OpenMP with all OS X gcc 4.2-based compilers" into release-4-6