The one on the registry page is horribly broken. Most functions are lacking the display parameter, there are some functions that don’t exist in the specification, and at the end of the file are some extension functions that don’t seem to belong there.
I found a (rather old) patch on sgi’s ogl-sample mailing list, which fixes most problems, but it seems unofficial. I also found the cvsweb interface for the sample implementation, but it does not appear to have any rules for building glx.h and instead has a static version.
So, is there a correct and up-to-date glx.spec somewhere or do I just need to hack my way with the broken one?