Enable VDPAU by default for mplayer on i386/amd64; general port cleanup
amd64/i386:
- Enable VDPAU by default for the large NVIDIA user base, proposed by ehaupt
- BUILD_DEPEND on yasm only on amd64/i386
General cleanups:
- Remove REINPLACE_CMD on CONFIGURE_SCRIPT conditional for post-8.x machines, is now part of patch-configure
- Offer RTCPU OPTION only on archs that support it (amd64, i386, ppc)
- Update RTCPU_DESC accordingly
- Use OPTION helper for RTCPU instead of complicated conditionals
- Remove superfluous check for ARCH when setting CFLAGS for armv6