libvorbis: use unversioned docdir

[ryandesign (Ryan Carsten Schmidt)]

libvorbis seems to have borrowed some of its portfile code from libogg:

post-destroot {
    set docdir ${prefix}/share/doc/${name}-${version}
    if {! [variant_isset doc]} {
        xinstall -d ${destroot}${docdir}
    }
    xinstall -m 0644 -W ${worksrcpath} AUTHORS CHANGES COPYING README \
        ${destroot}${docdir}
}

Problem is, libvorbis doesn't have a doc variant. It just always installs the additional documentation (as I've requested libogg also do in #31681). But it would be good to clean up this code in libvorbis, and also to install into an unversioned docdir, if you can figure out where to patch the makefile to do that.

[ryandesign (Ryan Carsten Schmidt)]

r86200.