You don't. Xfree86 source needs no patches at all. The source for fontconfig, freetype and
xft2 is separated from xfree86 (perhaps because Keith's leave). Slackware just added these sources into official xfree86 source tree and build them together. So you can still use firefly's patches on them, and add them into Xfree86.
An alternative is that you build separated freetype, fontconfig and xft2 packages, and install them on top of Slackware's xfree.