configure: fix pmi check

this commits fixes a regression introduced by open-mpi/ompi@75d8a7f25b467bd052b875a515b69bdbf98193a4
-I/usr/include/slurm is now added if needed to
 * pmix_CPPFLAGS
 * pmix_s1_CPPFLAGS
 * pmix_s2_CPPFLAGS