testing/gifsicle: new aport

gifsicle provides three tools:
gifsicle, which can animate/split/optimize/unoptimize GIF images;
gifdiff, which determines whether two GIFs appear identical; and
gifview, a simple viewer for animated GIFs.

gifview is the only one that uses X11, so it may be worthwhile to
split it out.
testing/gifsicle: build fix