m4/mpd_auto.m4: fix option description in MPD_ARG_ENABLE

Was using the wrong variable.  Regression by commit 16f870aa
configure.ac: improve description for --enable-id3
m4/mpd_with_flags: add macro MPD_WITH_FLAGS
configure.ac: use AC_CHECK_FUNC for Tremor check

Don't imply the library's name is "vorbisidec"; use whatever we found
inside TREMOR_LIBS.
configure.ac: make failure to find libvorbisidec fatal
configure.ac: reset tremor_prefix
configure.ac: inherit existing TREMOR_CFLAGS and TREMOR_LIBS

Replaces the old options --with-tremor-libraries and
--with-tremor-includes.
configure.ac: move --with-tremor declaration down
configure.ac: check for pthread before librt

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.
configure.ac: use AC_CHECK_FUNCS for pthread_setname_np()

Don't assume "-lpthread".  Just use PTHREAD_CFLAGS, as determined by
AX_PTHREAD.
configure.ac: add macro MPD_ENABLE_AUTO_LIB
configure.ac: use MPD_ENABLE_AUTO_LIB for libfaad