If I understand things correctly, the Emacs 'configure' script
discovered that the test glibc version did not declare and define a
symbol __malloc_initialize_hook, and so Emacs supplied its own
implementation of malloc, complete with __malloc_initialize_hook. Since
__malloc_initialize_hook was poisoned, this didn't work.
I suppose Emacs could work around the problem by using
__malloc_initialize_hook when linked against an old glibc, and by using
a new symbol emacs_malloc_initialize_hook when linked against its
substitute implementation. Although this would insulate distant-future
versions of Emacs against the poisoning, it wouldn't work for Emacs 25
(the next Emacs version) and earlier; these systems would be unbuildable
with a glibc that poisons __malloc_initialize_hook. So as a practical
matter, aren't we better off having glibc simply not declare
__malloc_initialize_hook?