malloc-gnu: Allow use as dependency from test modules.
* m4/malloc.m4 (gl_FUNC_MALLOC_GNU): Set REPLACE_MALLOC_FOR_MALLOC_GNU
instead of REPLACE_MALLOC.
(gl_FUNC_MALLOC_PTRDIFF, gl_FUNC_MALLOC_POSIX): Set
REPLACE_MALLOC_FOR_MALLOC_POSIX instead of REPLACE_MALLOC.
* m4/calloc.m4 (gl_FUNC_CALLOC_POSIX): Test
REPLACE_MALLOC_FOR_MALLOC_POSIX instead of REPLACE_MALLOC.
* m4/realloc.m4 (gl_FUNC_REALLOC_POSIX): Likewise.
* m4/stdlib_h.m4 (gl_STDLIB_H_REQUIRE_DEFAULTS): Initialize the
malloc-gnu module indicator.
(gl_STDLIB_H_DEFAULTS): Initialize REPLACE_MALLOC_FOR_MALLOC_GNU,
REPLACE_MALLOC_FOR_MALLOC_POSIX, not REPLACE_MALLOC.
* modules/stdlib (Makefile.am): Substitute GNULIB_MALLOC_GNU,
REPLACE_MALLOC_FOR_MALLOC_GNU, REPLACE_MALLOC_FOR_MALLOC_POSIX, not
REPLACE_MALLOC.
* modules/malloc-posix (Depends-on, configure.ac): Test
REPLACE_MALLOC_FOR_MALLOC_POSIX instead of REPLACE_MALLOC.
* modules/malloc-gnu (Comment): Remove section.
(Depends-on): Add xalloc-oversized.
(configure.ac): Test REPLACE_MALLOC_FOR_MALLOC_GNU instead of
REPLACE_MALLOC. Set a module indicator.
* lib/stdlib.in.h (malloc): Test REPLACE_MALLOC_FOR_MALLOC_POSIX,
REPLACE_MALLOC_FOR_MALLOC_GNU, and the respective module indicators
instead of just REPLACE_MALLOC.