x11-libs/libXvMC-1.0.1 can't find XvMCConfig. When an application attempts to initialize XvMC, the message "can't open XConfigDir/XvMCConfig" is shown. The problem appears to be the XVMC_CONFIGDIR is defined in Makefile.in, as though it should be substituted for the proper value, but it is not. In any case, the proper value for Gentoo should always be /etc/X11. Therefore, I'm attaching a patch to libXvMC and a patch to the ebuild which will apply it. The patch is tested and working here on amd64.
Created attachment 82834 [details, diff] Hardcode XVMC_CONFIGDIR This is always true on Gentoo; not sure where the current code works, but it isn't here.
Created attachment 82835 [details, diff] Update ebuild to apply patch Apply the patch in the previous attachment. ${PATCHES} is a variable defined in the x-modular eclass.
Fixed in -r1