Even if 'dist-8x8' is enabled with configure,
the dist-8x8 is not acutally enabled (so, no change in encoding behaviour)
until the command line option, '--enable-dist-8x8=1" is used.

The cdef-dist and daala-dist can not be enabled by a command line option yet.

This commit is a part of prep-work to remove DIST_8X8, CDEF_DIST,
and DAALA_DIST experimental flags.

Change-Id: I5c2df90f837b32f44e756572a19272dfb4c3dff4