xrootd: allow specification of C++ standard. (#10577)
xrootd: allow specification of C++ standard. (#10577)

* xrootd: add cxxstd variant.

* xrootd: support for recent and upcoming versions.

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