Don't scan tag items if the handler doesn't implement the tag()
method.

May be more clear than operators.

Don't assume "-lpthread".  Just use PTHREAD_CFLAGS, as determined by
AX_PTHREAD.

Works around a linker problem with some older glibc versions: if
"-lrt" was used, then "-pthread" was implied, but only the symbols
used by librt were available.  This led to a linker error because
pthread_atfork() was not found.

So with "-lrt", autoconf decides that no pthread flag is necessary,
but in the end fails due to pthread_atfork() missing.

By checking for pthread before librt, we avoid this dependency
problem.

Replaces the old options --with-tremor-libraries and
--with-tremor-includes.

Don't imply the library's name is "vorbisidec"; use whatever we found
inside TREMOR_LIBS.

Was using the wrong variable.  Regression by commit 16f870aa

Skipping those songs silently will confuse the client, because
commands specifying the song index within a playlist
(e.g. playlistdelete) will be out of sync.

This copies spl_print()'s behavior to playlist_file_print().