Unset F90 and F90FLAGS in mvapich2 (#8698)

* Unset F90 in mvapich2 when using intel compilers.

* Fixing typo.

* Unset F90 and F90FLAGS for mvapich2 at all times.

* Adding comment.

var/spack/repos/builtin/packages/mvapich2/package.py

@@ -199,6 +199,9 @@ def file_system_options(self):
 
     def setup_environment(self, spack_env, run_env):
         spec = self.spec
+        # mvapich2 configure fails when F90 and F90FLAGS are set
+        spack_env.unset('F90')
+        spack_env.unset('F90FLAGS')
         if 'process_managers=slurm' in spec:
             run_env.set('SLURM_MPI_TYPE', 'pmi2')