glide support is missing from our xfree ebuild. To learn how to add it, see the Slack sources: http://telia.dl.sourceforge.net/mirrors/slackware/slackware-8.0/source/x/XFree86/
Problem with glide, etc is that you need to have external libraries installed, so either create new use variables, or make X install with glide support by default. Another problem is that the glide source is not being maintained anymore, and near impossible to find since 3dfx closed down (relative new versions).
Availible in xfree85-4.2.0-r6