Change autoconf required version for libmar's configure.ac

Having it set to 2.68 (current for Ubuntu Precise) broke autoreconf for other
distributions. Notably Debian Squeeze/Wheezy have 2.67 and Ubuntu Lucid has
2.65 for their autoconf version.

Set a minimum requirement of 2.61 that we use in the existing options.

Signed-off-by: Ben Shum <bshum@biblio.org>