xsdk: phist has a conflict with gcc version <= gcc-4.9.1 (#11946)
xsdk: phist has a conflict with gcc version <= gcc-4.9.1 (#11946)

* xsdk: phist has a conflict with gcc version <= gcc-4.9.1 - so add a variant to disable it on older compiler builds

This is triggered by #11677

* fix typo

iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack