Issue an explicit warning if the docs can't be built at 'make dist'

Issue an explicit warning if the docs can't be built at 'make dist'
time, as is now required.

This is a little less broken than the previous config in that the dist
will need to have the build docs, and a normal build from a dist tarball
won't try. However if you --enable-docs on the dist tarball, it will
rebuild them even if they're up to date. Among other things, this makes
'make distcheck' annoying.

svn path=/trunk/vorbis/; revision=7816