build: Fix configure.ac for strict C99.
* configure.ac: Add function declarations for asm functions.
Cherry-pick master commit of:
693ffa145378682229473b0e811a9cea7c4d307a
- Suggested-by: Florian Weimer <fweimer@redhat.com>
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>