Use configure.ac hack to apply serial_test AM option only if supported.

Use configure.ac hack to apply serial_test AM option only if supported.

Inspired by libguestfs' configure.ac hack - test automake version,
and if 1.12 or newer, use m4 magic to pass "serial_tests" option
to AM_INIT_AUTOMAKE().

https://www.redhat.com/archives/libguestfs/2013-February/msg00102.html

Trac #427

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: David Sommerseth <davids@redhat.com>
Message-Id: <1433185487-9724-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9769
(cherry picked from commit c615835aa93701c764c23fc2579d97757c1a9970)