Add support for gfortran to be used with clang (#2192)

1) list gfortran as a fc and f77 compiler that can work with clang
2) allow compatible gfortran to ./spack compiler find with clang by matching version numbers

This is based on the discussions in

https://github.com/LLNL/spack/issues/237
https://github.com/dealii/dealii/wiki/deal.II-in-Spack#mixing-gcc-and-clang-on-osx

This is not a long term solution but something to get us through the next months until the compiler
infrastructure is reworked to allow mixing and matching for C/C++ and Fortran compilers

Funded-by: IDEAS
Project: IDEAS/xSDK
Time: 1.5 hours