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 <dd437fc32dcda8cbb48d47baa844ad777493d02b@samsung.com>
Reported-By: Vinson Lee <25a259f709b0f1b620605d2518271a846d8826e5@freedesktop.org>
Tested-by: Bryce Harrington <dd437fc32dcda8cbb48d47baa844ad777493d02b@samsung.com>
Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>

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