backport OPAL_WANT_MEMCHECKER macro

This fixes commit open-mpi/ompi-release@3156f98d358a5d3f34e21324a11708441153268f
which is a cherry pick from open-mpi/ompi@a422d893b8e63347deb71bbaaa14ca25ddb1f192

@jsquyres could you please review this PR ?