configure.ac: Add swig dependency

configure.ac: Add swig dependency

As far as I can tell (and recommended to me by Matt) taking these m4
extension macros from http://www.gnu.org/software/autoconf-archive/
doesn't require the project distribute GPL. I am a bit confused from
reading the license. I'd really hope someone can comment.

The only other solution would be to roll my on m4 macros, or figure out
a way to check that this autoconf-archive package is included from the
configure.ac.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>