if you don't explicitly call it, then it is implied by the first use of
PKG_CHECK_MODULES (and no later ones).  This results in the check only
being done in the test for pulse, which is skipped when --disable-pulse is
used.  This will fix the bug mentioned in the previous commit.

git-svn-id: https://svn.musicpd.org/mpd/trunk@5142 09075e82-0dd4-0310-85a5-a0d7c8717e4f