--- a/make/Help.gmk	Wed Oct 10 23:05:15 2018 +0200
+++ b/make/Help.gmk	Mon Oct 15 11:36:20 2018 -0700
@@ -119,7 +119,7 @@
 run-test-prebuilt:
 	@( cd $(topdir) && \
 	    $(MAKE) --no-print-directory -r -R -I make/common/ -f make/RunTestsPrebuilt.gmk \
-	    run-test-prebuilt TEST="$(TEST)" )
+	    run-test-prebuilt CUSTOM_MAKE_DIR=$(CUSTOM_MAKE_DIR) TEST="$(TEST)" )
 
 ALL_GLOBAL_TARGETS := help print-configurations run-test-prebuilt