Warn at configure time if pixman/cairo/freetype/etc are not the same as the test suite expects
@ptomato came up with a nice idea: check in configure.ac if the versions of pixman, cairo, freetype, and pango are the same as the ones used to generate the reftest images, and print a warning during configure if not.
This would certainly serve as a reminder that things may go amiss in the test suite. See #505 (closed) for the bigger-picture view.
Example code at https://gitlab.gnome.org/GNOME/gjs/blob/5872749d/configure.ac#L374-384