FontForge is an outline font editor that lets you create your own postscript, 
truetype, opentype, cid-keyed, multi-master, cff, svg and bitmap (bdf, FON, 
NFNT) fonts, or edit existing ones. It also lets you convert one format to 
another. FontForge has support for many Macintosh font formats.

You can specify an alternate version of freetype on the command line when 
running the script with e.g.   FREETYPE=2.3.12 ./fontforge.SlackBuild
but you will need to manually place that freetype tarball in the slackbuild 
directory.
