configure.ac: avoid the rabbitmq configure script if Python 2.5 is not available

Because that causes the rabbitmq configure script to fail with a difficult
to diagnose error message.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Merge pull request #158 from balabit/f/dont-configure-rabbitmq-without-python

configure.ac: avoid the rabbitmq configure script if Python 2.5 is not available