[gcc] Make ISL optional by adding a +graphite variant (#20531)
[gcc] Make ISL optional by adding a +graphite variant (#20531)

Fixes: #20515

Co-authored-by: Baptiste Jonglez <46f1a0bd5592a2f9244ca321b129902a06b53e03@bitsofnetworks.org>
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack