Autoconf is missing --with-flac parameter
On custom installs, vorbis-tools (version 1.4.0) may not be able to locate flac when you run the ./configure script. However, running ./configure --help reveals that the only option related to flac is: --without-flac
It would be useful (and a fairly standard way of doing things in autoconf) if the option were provided to specify the location of flac when it can't be located in the usual locations, with a specifier such as --with-flac