include-next, warnings: support older autoconf

include-next, warnings: support older autoconf

Older autoconf did not know how to peer through quoting of the
m4 macro created by AS_VAR_PUSHDEF; while newer autoconf has
been patched to allow typical autoconf quoting rules to still
apply, compatibility with autoconf 2.59 requires the unquoted
use of the macro name for all uses prior to AS_VAR_POPDEF.

absolute-header.m4 and warn-on-use.m4 already got this right.

* m4/include_next.m4 (gl_NEXT_HEADERS_INTERNAL): Use
AS_VAR_PUSHDEF in a way that works with older autoconf.
* m4/warnings.m4 (gl_WARN_ADD): Likewise.
Reported by Daniel P. Berrange.

Signed-off-by: Eric Blake <eblake@redhat.com>