I don't think that a patch be necessary as none is used to make the mesa-8.0.4 package available in Slackware-current.
I suggest that instead of compiling it manually you just make a Slackware package for mesa-8.0.4, using the SlackBuild available in Slackware-current in /source/x/mesa.
PS I confirm that the mesa-8.0.4 package installs nouveau_device.h in /usr/include/nouveau/
Last edited by Didier Spaier; 08-31-2012 at 03:54 AM.
Reason: PS added