build: Remove support for non-pkg-config Expat

The Expat XML library has shipped a pkg-config file for long enough to
be in Debian's oldstable (Jessie, April 2015) and Ubuntu's oldest
supported LTS (Trusty, 14.04). The pkg-config file was added in Expat
upstream's commit 352cfc8f59a7, in September 2007.

Drop build support for versions of Expat which do not ship a
pkg-config file.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derek.foreman.samsung@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>