hpcc: add explicit C99 flag for older GCC versions (#18556)
hpcc: add explicit C99 flag for older GCC versions (#18556)


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