Fixed by https://github.com/liblouis/liblouis/pull/324
which has been merged upstream.
diff --git tools/brl_checks.c tools/brl_checks.c
index c125ed5b..0ef2f808 100644
--- tools/brl_checks.c
+++ tools/brl_checks.c
@@ -13,6 +13,7 @@ are permitted in any medium without royalty provided the copyright
 notice and this notice are preserved. This file is offered as-is,
 without any warranty. */
 
+#include <config.h>
 #include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
