configure.ac: print feature name in error message

When MPD_AUTO_DISABLED prints a fatal error message, include the
feature name.  This might be an important piece of information for the
user, just in case the preceding line doesn't tell him.

m4/mpd_auto.m4

@@ -17,7 +17,7 @@ AC_DEFUN([MPD_AUTO_DISABLED], [
 		AC_MSG_WARN([$msg -- disabling $feature])
 		eval "$var=no"
 	elif eval "test x`echo '$'$var` = xyes"; then
-		AC_MSG_ERROR([$msg])
+		AC_MSG_ERROR([$feature: $msg])
 	fi
 ])