Exclude VP9 assemblies from VP8 builds
Exclude VP9 assemblies from VP8 builds

Add a macro to to exclude VP9 specific assembly files from build if VP9
is not configured. This would otherwise cause a linking error for VP8
only builds.

BUG=webm:1625

Change-Id: I6d892b7c2837a2574538d18b776fd2b6d706da96

ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx