PKG_PROG_PKG_CONFIG to be done first.  But this is only done if PulseAudio
and JACK aren't explitily disabled.  Therefore, using ./configure
--disable-pulse --disable-jack will make it so that neither LSR nor
Zeroconf can be found.  This is the second time this bug has come up (first
being when JACK was added), so can people please start adding to the test
around PKG_PROG_PKG_CONFIG when they add new features that need
PKG_CHECK_MODULES?

git-svn-id: https://svn.musicpd.org/mpd/trunk@5453 09075e82-0dd4-0310-85a5-a0d7c8717e4f