Instead of unconditionally setting the memory hook, only set it when
the memory hooks are both available and have been enabled (e.g.,
opal/mca/memory/linux has decided that it *can* be enabled, and when
the mpi_leave_pinned MCA param is set to 1, or is set to -1 and some
component requested the memory hooks be enabled).
If we set the memory hook when memory hooks are not enabled,
__malloc_hook will be NULL, which will cause problems when
btl_openib_malloc_hook() tries to invoke it.
Fixesopen-mpi/ompi#638.
88c13adc8c