ive noticed a few packages have optional support for the graphic format 'xpm' ... the only one i can cite right now is 'libgd' ... but i know there are others because i've either ignored them in the past or hardcoded support because i didnt think it was widely used ...
*** This bug has been marked as a duplicate of 1446 ***
the use flag should be covered by the X variable instead.
it may not even be necessary for xpm to be optional -- if the package depends on X, build xpm
will do ;)