I figured it out, I think. I forgot to copy the plugins. So now my steps are
mkdir firefly
cd firefly
wget http://nightlies.fireflymediaserver.org/nightlies/svn-1586/mt-daapd-svn-1586.tar.gz
tar -xvzf mt-daapd-svn-1586.tar.gz
cd mt-daapd-svn-1586
./configure --enable-flac --enable-sqlite3 --with-id3tag=/opt/local
make
cp src/mt-daapd /Library/PreferencePanes/Firefly.prefPane/Contents/Resources/Firefly Helper.app/Contents/Resources/Server/firefly/
cp src/plugins/.libs/*.so /Library/PreferencePanes/Firefly.prefPane/Contents/Resources/Firefly Helper.app/Contents/Resources/Server/plugins/Rendezvous discovery is working.
Also I think my patch (the whole reason I’m doing this) is working, but will test further and then post details.