Creates a merge between the master and experimental branches. Fixes a
number of conflicts in the build system to allow *either* VP8 or VP9
to be built. Specifically either:

  $ configure --disable-vp9 $ configure --disable-vp8
  --disable-unit-tests

VP9 still exports its symbols and files as VP8, so that will be
resolved in the next commit.

Unit tests are broken in VP9, but this isn't a new issue. They are
fixed upstream on origin/experimental as of this writing, but rebasing
this merge proved difficult, so will tackle that in a second merge
commit.

Change-Id: I2b7d852c18efd58d1ebc621b8041fe0260442c21