Add Fortran compiler variants

Fortran compiler variants have been added. C++ code is now compiled with
clang, as recommended for Mavericks:
http://trac.macports.org/wiki/FAQ#libcpp
Make the changes suggested here: https://trac.macports.org/ticket/42886

The Portfile has been simplified based on the comments on the
tracker. In addition, several unsupported compilers have been
blacklisted. Finally, we now use the gcc's cpp, as recommanded by
Gildas developers.