buildconf: exec autoreconf to avoid additional process
Also make buildconf exit with the return code of autoreconf.
Follow up to
@@ -5,4 +5,4 @@ echo "*** Do not use buildconf. Instead, just use: autoreconf -fi" >&2
echo "*** Doing it for you now, but buildconf may disapear in the future." >&2
echo "***" >&2
-${AUTORECONF:-autoreconf} -fi "${@}"
+exec ${AUTORECONF:-autoreconf} -fi "${@}"
