automake: properly handle non-default expat installation

Use PKG_CHECK_MODULE over requesting the user to setup the
option at configure time. Drop unused EXPAT_INCLUDE and
update all targets.

NOTE: The this commit removes the --with-expat configure
option. One should ensure that the expat they wish to use
has expat.pc file accessible by pkg-config.

v2:
* Add note about the removal of --with-expat
(per Tom Stellard)
* Drop EXPAT_CFLAGS for targets that do not build DRI_COMMON
(spotted by Matt Turner)

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>