hwloc/configury: fix backport to the v1.8 branch

use the OMPI_CFLAGS_BEFORE_PICKY variable instead of the
OPAL_CFLAGS_BEFORE_PICKY (which is good for master only)
Merge pull request #146 from ggouaillardet/topic/hwloc-fix-backport

hwloc/configury: fix backport to the v1.8 branch