flac: fix buildpaths warnings

Generated documentation (html) contain absolute paths cources
using buildpaths warnings.
Replace them with relative links.

The file with root path to sources is in my build
/usr/share/doc/flac/api/dir_c122f5d6544f32779f55e8358fb78605.html
which does not looks as stable name, so replace it in all files.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit c7d826c8)
Signed-off-by: Steve Sakoman <steve@sakoman.com>