I made a couple of fixes in git... they *might* affect your situation (although
TBH I'd be a little surprised). Can you test it out with a mesa from git?

Note that LD_LIBRARY_PATH has no effect on which libvdpau_nouveau.so is
loaded... make sure to set VDPAU_DRIVER_PATH (and have a new enough libvdpau
that supports that... 0.8 iirc). Or install a libvdpau into the same prefix
that the mesa install goes into, then LD_LIBRARY_PATH should do what you
expect.