Fix for m4%clang (#4912)
Fix for m4%clang (#4912)

* Fix for m4%clang

* Restricted condition to not subsitute rtlib on OSX

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