Fixes #259: Apple clang compiler detection
Fixes #259: Apple clang compiler detection

- Clang compilers are now versioned with the raw LLVM version or, on
  macs, with Apple's version with a suffix.

lgarren/spack,lgarren/spack,matthiasdiener/spack,lgarren/spack,TheTimmy/spack,matthiasdiener/spack,EmreAtes/spack,LLNL/spack,EmreAtes/spack,lgarren/spack,LLNL/spack,mfherbst/spack,krafczyk/spack,mfherbst/spack,TheTimmy/spack,iulian787/spack,krafczyk/spack,mfherbst/spack,mfherbst/spack,tmerrick1/spack,EmreAtes/spack,iulian787/spack,iulian787/spack,krafczyk/spack,skosukhin/spack,tmerrick1/spack,krafczyk/spack,matthiasdiener/spack,TheTimmy/spack,skosukhin/spack,matthiasdiener/spack,LLNL/spack,tmerrick1/spack,krafczyk/spack,TheTimmy/spack,iulian787/spack,EmreAtes/spack,lgarren/spack,tmerrick1/spack,skosukhin/spack,mfherbst/spack,LLNL/spack,matthiasdiener/spack,tmerrick1/spack,skosukhin/spack,skosukhin/spack,TheTimmy/spack,LLNL/spack,iulian787/spack,EmreAtes/spack