configure.ac: added the --enable-sqlite option

MPD will (optionall) use sqlite databases in the future.  Add a
configure option to enable that.  There is no code yet to really use
sqlite, so the practical use of this patch is limited.

src/Makefile.am

@@ -2,6 +2,7 @@ bin_PROGRAMS = mpd
 
 mpd_CFLAGS = $(MPD_CFLAGS)
 mpd_CPPFLAGS = \
+	$(SQLITE_CFLAGS) \
 	$(CURL_CFLAGS) \
 	$(AO_CFLAGS) $(ALSA_CFLAGS) \
 	$(SHOUT_CFLAGS) \
@@ -14,6 +15,7 @@ mpd_CPPFLAGS = \
 	$(FFMPEG_CFLAGS) \
 	$(GLIB_CFLAGS)
 mpd_LDADD = $(MPD_LIBS) \
+	$(SQLITE_LIBS) \
 	$(CURL_LIBS) \
 	$(AO_LIBS) $(ALSA_LIBS) \
 	$(SHOUT_LIBS) \