Clang has fully implemented OpenMP support as of LLVM 3.7, so if OpenMP
is required by a formula gcc is no longer the only choice of compiler.

Clang should be preferred over gcc because using gcc meant linking
against libstdc++, which is ABI incompatible with libc++. This may be
unnoticeable for some users, but it causes other builds to fail, e.g.
pstoedit when imagemagick was built with OpenMP. pstoedit is required
for the octave formula, so for some users this could be a significant
problem.