Tentative fix for issue #89.

Try to be more generic in the configure script when checking for funky gfortran compiler names.

@astrofrog Can we assume ``sed`` is available on all supported platforms? Could you check on OSX please? :)

Also, not sure it is necessary to change the ``configure`` script, as it is generated from the .ac. But since it is included in the repo I thought I would change it as well.