Addition of a ‘perf’ variant to selected ports

[Vincent Habchi]

Folks,

I am beginning to patch some ports to add a special variant called ‘perf’ that picks clang-3.3 and dragonegg-3.3-gfortran-4.8 (if necessary) instead of standard compilers then replace standard flags by ‘-O3 -march=native’. The idea is to provide a way for users that seek performance to optimize the executable code by selecting the most recent compilers and aggressive optimization flags (the above combination enables loop vectorizing and AVX instructions if possible). Obviously, this is only relevant to some math or science ports; possible candidates include: metis, cgal, boost (?), geos, atlas, arpack…

Since this option results in non-portable code (because of the -march=native flag) it will always be disabled by default, so the buildbot can carry on cranking out widely compatible code.

Thoughts?

Cheers!
Vincent