configure.ac: Fallback to older detection code if pkg-config can't find expat

This paritally reverts commit a4afd90f.

On older expat versions (ie the one on Ubuntu 12.04) there is no pkg-config
file, so fall back to a test for the header and library. In addition the
source for expat does not seem to be in a git repository but in cvs instead
and it seems preferrable to not require cvs to build wayland.

The restored test has been updated to use AC_SEARCH_LIBS. This version
uses empty square brackets for the unused branches, similar to many other
if statements in configure.ac.
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Andrew Oakley <aoakley@espial.com>