configure.ac: improve database dependency checks

Abort if --enable-libmpdclient or --enable-upnp are used with
--disable-database, instead of ignoring the mismatch silently.

m4/mpd_depends.m4

@@ -0,0 +1,9 @@
+AC_DEFUN([MPD_DEPENDS], [
+	if test x$$2 = xno; then
+		if test x$$1 = xauto; then
+			$1=no
+		elif test x$$1 = xyes; then
+			AC_MSG_ERROR([$3])
+		fi
+	fi
+])