Allow disabling disabled codecs

When using 'make dist' after --disable-vp8[encoder|decoder] it would
fail to recognize the option. This would only occur when also specifying
--enable-install-docs and --enable-install-srcs but not
--enable-codec-srcs

Including vpx/ fixes builds with --enable-codec-srcs

vpx_timer.h is also required for vpxenc.c

Change-Id: Ie3e28b2f7ec7ee6d5961d3843f9eab869f79c35b