Default REL_ALLOC to 'no'

Default REL_ALLOC to 'no'

This should make ralloc-related bugs less likely on GNU/Linux
systems with bleeding-edge glibc.  See the email thread containing:
http://lists.gnu.org/archive/html/emacs-devel/2016-10/msg00801.html
Do not merge to master.
* configure.ac (REL_ALLOC): Default to 'no' on all platforms, not
merely on platforms with Doug Lea malloc.  Although bleeding-edge
glibc no longer exports __malloc_initialize_hook and so longer
passes the configure-time test for Doug Lea malloc, ralloc tickles
longstanding bugs like Bug#24358 and Bug#24764 and Emacs is likely
to be more reliable without it.  This patch is not needed on
master, which uses hybrid malloc in this situation.