As said in the summary, if I compile vdpau-video-0.7.4 with libvdpau-0.8 it fails with the following error message:

libtool: compile:  x86_64-pc-linux-gnu-gcc -DHAVE_CONFIG_H -I. -march=native -pipe -O2 -floop-block -floop-interchange -floop-strip-mine -c vdpau_decode.c  -fPIC -DPIC -o .libs/vdpau_decode.o
libtool: compile:  x86_64-pc-linux-gnu-gcc -DHAVE_CONFIG_H -I. -march=native -pipe -O2 -floop-block -floop-interchange -floop-strip-mine -c vdpau_dump.c  -fPIC -DPIC -o .libs/vdpau_dump.o
libtool: compile:  x86_64-pc-linux-gnu-gcc -DHAVE_CONFIG_H -I. -march=native -pipe -O2 -floop-block -floop-interchange -floop-strip-mine -c vdpau_driver.c  -fPIC -DPIC -o .libs/vdpau_driver.o
Makefile:468: recipe for target 'vdpau_decode.lo' failed
vdpau_decode.c: In function 'vdpau_EndPicture':
vdpau_decode.c:1295:9: error: invalid use of void expression
         );
         ^
make[2]: *** [vdpau_decode.lo] Error 1
make[2]: *** Waiting for unfinished jobs....

Gentoo has a patch [1] which they grabbed from FreeBSD according to [2].

[1] http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/x11-libs/libva-vdpau-driver/files/libva-vdpau-driver-0.7.4-libvdpau-0.8.patch?view=markup
[2] https://bugs.gentoo.org/show_bug.cgi?id=517778