Have R extensions build in parallel.
Have R extensions build in parallel.

Set `MAKEFLAGS` so R extensions can be built in parallel if that is set
in spack.

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