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().

Was hard-coded to "auto", and the macro parameter was ignored (in the
help text).

If no name is specified, it'll use the name of the first activity of the
AndroidManifest.xml. And we don't want the apk to be called Settings.apk.

Not used.