Prefer system VDPAU headers if present

[turboencabulator]

In case they happen to be different from the bundled ones.

[i-rinat]

I don't think this is a good idea. I can't see this change helping/fixing/simplifying anything, but at the same time it may cause issues. In fact this approach did cause issues before.

Initially, I used VDPAU headers from libvdpau-dev, and it was fine. But at some moment I've found that mpv tried to use Constrained Baseline profile of H.264 (#50 (comment)). Any hardware that supports higher profiles automatically supports Baseline and Constrained Baseline. It's just libvdpau-va-gl wasn't checking VDP_DECODER_PROFILE_H264_CONSTRAINED_BASELINE. The fix is trivial, but it caused compilation failures on machines with older versions of libvdpau-dev: #55.

Workaround may be possible, but I did not manage to come up with one that would work with all versions of VDPAU headers. So the only solution left was to use a specific version of headers. There should be no issues with older code on newer systems, since VDPAU extends its structures by appending members and never rearranging older ones.