Fix mumps compilation with gcc@10 (i.e., gfortran) (#21631)
Fix mumps compilation with gcc@10 (i.e., gfortran) (#21631)

* add new flag when compiling mumps with %gcc@10.

* Fix style

* Try to fix formatting

* Use flag_handler approach suggested by @michaelkuhn
	in the PR review.

* Delete former approach

* Another style issue

* Add another space

* More fixes
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack