gcc: don't configure with version suffix.

Just add symlinks for gcc, g++ for backwards compatibility.
gcc: build gfortran by default.
fortran_dependency: use gcc instead of gfortran.
Remove gfortran in favour of gcc formula.
gfortran: alias from gcc.
gcc: bump revision for version suffix changes.
git-annex: use unversioned gcc binary name.
libxc: bump revision to test gfortran changes.