The hardest part is determining $VERSION numbers for each module. Last I checked, Slackware was doing rolling-current releases. Meaning, using development versions intended for the next stable release. I tried that for quite a few months at first but it's way too problematic so I started to build stable $VERSIONS only as defined by the Xorg Foundation.
http://xorg.freedesktop.org/releases/
I'm also not remotely interested in modular packages so I build monolithic 'package sets'. That leaves me with these packages for Xorg:
dejavu-fonts-ttf-2.23-noarch-1.tgz
fontconfig-2.5.0-i686-1.tgz
freetype2-2.3.5-i686-1.tgz
libdrm-2.3.0-i686-1.tgz
mesa-7.0.2-i686-1.tgz
ttf-bitstream-vera-1.10-noarch-1.tgz
xorg-app-7.3-i686-5.tgz
xorg-data-7.3-noarch-1.tgz
xorg-driver-7.3-i686-1.tgz
xorg-font-7.3-i686-1.tgz
xorg-lib-7.3-i686-3.tgz
xorg-proto-7.3-noarch-1.tgz
xorg-server-1.4-i686-3.tgz
xorg-util-7.3-i686-1.tgz
With each Xorg version, someone different is assigned to release, and up to this point, uploading the correct module versions is nothing more than an after thought. I was told, in a nut shell, by Alan C @ Xorg that they couldn't be bothered with having the correct version modules listed at the above link because they were too busy with the actuall development to care about such things. It's up to the packagers to find out what builds with what apparently.
It's like going to the abiword ftp site and finding abiword-2.6.4, abiword-plugins-2.6.2, abiword-docs-2.6.3 and abiword-extras-2.6.1 all in the same 2.6.4 release directory.
Anyway... My point being that's it's frustrating to build for that reason alone. Your going to need an automated way to grab all tarballs (and correct versions) otherwise you might as well forget about it. I wrote
picklist for just this purpose so it may be of use to you. I know Vector uses it and possibly others.
Dan N @ Xorg told me that he would try to crack the whip with each release upload to get all this stuff sorted. He has a vested interest in things being correct as well since he's also a developer @ {B}LFS. (think wget files).