Sorry, not a src2pkg problem either, I think. The sources are failing to compile: 'I915_EXEC_BLT' undeclared
Probably missing some headers somewhere. Do you have the proper X 'proto' packages installed?
I found only one other reference to the problem, here:
which shows the exact same errors as you are getting.
searching for just 'I915_EXEC_BLT' shows only about 10 results, which indicate that this is brand new code in libdrm-2.4.22. There are often conflicts between the headers in the drm sources and those which come with the kernel. There was a lot of discussion about this a year or two ago. I'm not sure what Slackware wound up doing about it, if anything, but it used to ship with the headers as they come with the kernel sources. The kernel is supposed to ship with the lates drm headers -but that means the very latest kernel. Anyway, you'll probably have to compile and install drm first in order to get use of that new code. Be aware that doing so will overwrite the headers shipped with the Slackware kernel-headers package. And the new headers may interfere with compiling older sources which depend on drm.
Here are a few sites which reference 'I915_EXEC_BLT':
Found in: linux-2.6.37-rc3-ccs-1.8.0:
Found in drm mailing list: