Assert that the broken symlink is actually overwritten
Assert that Keg#link produces relative symlinks
curl: explicitly disable unwanted libraries

curl will opportunistically pick up unrequested librares, despite
superenv filtering. This adds a few extra options for things curl can
find on its own, and explicitly disables anything that's unrequested.
Update gifsicle and make X11 dependency optional
gifsicle: add 1.81 bottle.