Supply malloc and alloc_size attributes for extern allocators.

Supply malloc and alloc_size attributes for extern allocators.

This documents the C API, and helps GCC generate a bit better code.
* conf_post.h (ATTRIBUTE_MALLOC, ATTRIBUTE_ALLOC_SIZE)
(ATTRIBUTE_MALLOC_SIZE): New macros.
* gmalloc.c (malloc, realloc, calloc):
* gtkutil.h (malloc_widget_value):
* lisp.h (ralloc, r_re_alloc, xmalloc, xzalloc, xrealloc, xnmalloc)
(xnrealloc, xstrdup, xlispstrdup, record_xmalloc):
Use them.