Don't use pkg-config's --atleast-version arg when checking for pango

It doesn't seem to work with some versions of pkg-config, e.g. the one
found on Ubuntu 10.04

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56327 75b3775d-3801-0410-9e12-e2e43121521d