Make Boost build with MPI on OS X
Make Boost build with MPI on OS X

Boost does not build on OS X with either gold or binutils. The gold linker does not exist on Darwin, and binutils on Darwin provides an assembler that doesn't work for Boost.

- Introduce a variant that specifies whether to build with binutils, defaulting to true for backward compatibility
- Auto-detect whether we build on Darwin; in this case, set the gold and binutils variant defaults to false
- Clean up configure flags for as and ld

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