bug#23760: 25.0.95; emacs 25.0.95 doesn't build with glibc-2.23.90

[Paul Eggert <eggert@cs.ucla.edu>]

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? That should work with older Emacs versions, 
which is a win. I don't see a significant practical advantage to 
poisoning the symbol.