automake: handle expat version pre 2.1

automake: handle expat version pre 2.1

Commit aec20d66d9d13e0acd6a7199b63e1383e1e9900a
(automake: properly handle non-default expat installation),
assumed that up-to date distributions use a recent version
of expat that handles security vunerabilities CVE-2012-1147
and CVE-2012-1148. Seems like this is not always the case
and they prefer to backport only the fix, rather than use
the updated library.

This commit adds a default case -lexpat whenever expat is
not found, while properly handling expat.pc if present.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71022
Reported-By: Bryce Harrington <b.harrington@samsung.com>
Reported-By: Vinson Lee <vlee@freedesktop.org>
Tested-by: Bryce Harrington <b.harrington@samsung.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>