Include <malloc.h> when advisable

Include <malloc.h> when advisable

This should help insulate us better from future glibc changes.
It is good hygiene to include .h files for APIs that Emacs uses.
Fix type clashes between Emacs and GNU <malloc.h> (Bug#22086).
* configure.ac: Check for malloc.h.
* src/alloc.c: Include <malloc.h> depending on HAVE_MALLOC_H,
not on DOUG_LEA_MALLOC.
* src/emacs.c, src/gmalloc.c (malloc_enable_thread):
Remove decl (now in lisp.h).
* src/gmalloc.c: Include stddef.h earlier, for ptrdiff_t.
[emacs]: Include lisp.h.
[HAVE_MALLOC_H]: Include <malloc.h>.
(__MALLOC_HOOK_VOLATILE): New macro, if not already defined.
(__after_morecore_hook, __malloc_initialize_hook, __morecore)
(__default_morecore):
[!HAVE_MALLOC_H]: New decls near non-inclusion of <malloc.h>.
(calloc): Make it clear that the macro should not be used.
Remove unused decl.
(malloc_info): New macro, to avoid clash with glibc <malloc.h>.
(__morecore, __default_morecore, __after_morecore_hook)
(__malloc_extra_blocks, __malloc_initialize_hook, __free_hook)
(__malloc_hook, __realloc_hook, __memalign_hook, memory_warnings):
Remove later decls.
(gmalloc_hook, gfree_hook, grealloc_hook):
Rename from __malloc_hook, __free_hook, __realloc_hook to
avoid type collision with glibc <malloc.h>.  All uses changed.
(gmalloc_hook):
(__malloc_extra_blocks) [DOUG_LEA_MALLOC||HYBRID_MALLOC||SYSTEM_MALLOC]:
Now static.
(gmalloc_hook, __malloc_extra_blocks): Define even if [!HYBRID_MALLOC].
(__malloc_initialize_hook, __after_morecore_hook):
Declare with types compatible with glibc.
(__memalign_hook, hybrid_calloc) [HYBRID_MALLOC]:
Remove.  All uses removed.
* src/lisp.h (__malloc_extra_blocks, malloc_enable_thread): New decls.
* src/ralloc.c, src/vm-limit.c:
Simplify includes and include <malloc.h> if available.