Avoid some more autoconf warnings.

* m4/acl.m4 (gl_FUNC_ACL): s/AC_HELP_STRING/AS_HELP_STRING/.
* m4/afs.m4 (gl_AFS): Likewise.
* m4/gc-random.m4 (gl_GC_RANDOM): Likewise.
* m4/absolute-header.m4 (gl_ABSOLUTE_HEADER): s/AC_FOREACH/m4_foreach_w/.
* m4/stdint.m4 (gl_STDINT_BITSIZEOF, gl_CHECK_TYPES_SIGNED)
(gl_INTEGER_TYPE_SUFFIX): Likewise.
* m4/onceonly_2_57.m4 (AC_CHECK_HEADERS_ONCE, AC_CHECK_FUNCS_ONCE)
(AC_CHECK_DECLS_ONCE): Likewise.
Rename file...
* m4/onceonly.m4: ...to this, and delete 2.54 variant, now that
gnulib-tool requires autoconf 2.59 or better.
* gnulib-tool (func_get_filelist): s/\(onceonly\)_2_57.m4/\1.m4/.

Signed-off-by: Eric Blake <ebb9@byu.net>