configure.ac: Improve the bison version check

Some versions of bison have "-" in their version number too, allow that
too, when verifying that the bison installed is a recent version.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #48 from balabit/merge/h/improved-yacc-check

configure.ac: Improve the bison version check