configure.ac: Fix publican version check

This sure is ugly - we feed output of publican -v into bc to compare
against minimum required version, 2.8.  That's bad enough, but when
publican suddenly report 3.0.0, bc starts complaining...

Use sed to filter out 3.0 from the 'version=3.0.0' output from publican
instead.  Seem a little more robust, but it's just a matter of time before
something else breaks this flaky setup.

Hey, publican, how about shipping .pc files?