Formula for xcftools

Here's a formula for Henning Makholm's `xcftools`, a package for manipulating [the GIMP's](http://www.gimp.org) native [XCF file format](http://docs.gimp.org/2.8/en/gimp-export-dialog.html).

This software is now used by [the imread python module](https://github.com/luispedro/imread) and the formula allows it to be easily built.

http://henning.makholm.net/software