configure.ac: udev basedir should be relative to prefix

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

configure.ac

@@ -62,7 +62,7 @@ AC_ARG_WITH(udev-base-dir, AS_HELP_STRING([--with-udev-base-dir=DIR], [where ude
 if test -n "$with_udev_base_dir" ; then
 	UDEV_BASE_DIR="$with_udev_base_dir"
 else
-	UDEV_BASE_DIR="/usr/lib/udev"
+	UDEV_BASE_DIR="$libdir/udev"
 fi
 AC_SUBST(UDEV_BASE_DIR)