RFC: Renaming GCC ports and variants

[Sean Farley]

Proposal:

Since it seems that we are flat-out disallowing gcc being used as a
C/C++ compiler, I think it's time to do some clean up of the code:

1) Rename gccXY to gcc-X.Y

2) Rename +gccXY variants to +gfortranXY

3) Start moving away from configure.compiler=macports-gcc*

I think it's important for (2) to happen because it make the variant
clear as to what it is adding (namely, just a fortran compiler).

There are about ~100 ports with gcc variants / configure.compiler being
set so, while not being trivial, it's not *that* bad.

Due to development burden, I think this kind of decision needs to be
across the board (e.g. not be conditional on the OS used). For this
proposal, it will hopefully clear up some C++ linking bugs.

As a first wave, these would be some ports to tackle:

atlas
dotwrp
julia
mpich-*
octave
openmpi-*
petsc
py-scipy
qrupdate
slepc

And, of course, update the compilers and mpi portgroups to use the
configure.compiler specified. If we can agree on a plan, I can volunteer
to do this work and submit my work to be reviewed before committing.

Comments welcomed.