backport OPAL_WANT_MEMCHECKER macro

This fixes commit open-mpi/ompi-release@3156f98d358a5d3f34e21324a11708441153268f
which is a cherry pick from open-mpi/ompi@a422d893b8e63347deb71bbaaa14ca25ddb1f192
Fix an other typo introduced by transfer of trunk change to v1.8
and not fixed by open-mpi/ompi-release@c7e9cf0a3cf261974ef082c07f02330112e87f01
Merge pull request #49 from ggouaillardet/topic/backport_vader

Fix an other typo introduced by transfer of trunk change to v1.8
Merge pull request #48 from ggouaillardet/topic/backport_opal_want_memchecker

backport OPAL_WANT_MEMCHECKER macro
Merge branch 'v1.8' of github.com:open-mpi/ompi-release into topic/sync_with_ompi_v1.8