Switch to using freetype-config for freetype detection.

This fixes GD compilation against libfreetype 2.5.1 and later after they made
the rather interesting decision to change their include directory layout in a
point release.

The original suggestion in the bug was to use pkg-config, but my inclination is
to use freetype-config instead: we should be able to get the same configuration
information without actually needing pkg-config installed, since pkg-config is
by no means guaranteed to exist on many Unices and distros, whereas
freetype-config should always be present if a libfreetype build environment is
installed. Let's try it out and see what happens.

Fixes bug #64405 (Use freetype-config for determining freetype2 dir(s)).