--- a/make/Init.gmk	Wed Oct 21 18:38:29 2015 -0700
+++ b/make/Init.gmk	Thu Oct 22 15:54:15 2015 +0200
@@ -137,7 +137,9 @@
 
     # The spec files depend on the autoconf source code. This check makes sure
     # the configuration is up to date after changes to configure.
-    $(SPECS): $(wildcard $(topdir)/common/autoconf/*)
+    CUSTOM_CONFIG_DIR ?= $(topdir)/closed/autoconf
+
+    $(SPECS): $(wildcard $(topdir)/common/autoconf/*) $(wildcard $(CUSTOM_CONFIG_DIR)/*)
         ifeq ($(CONF_CHECK), fail)
 	  @echo "Error: The configuration is not up to date for '$(lastword $(subst /, , $(dir $@)))'."
 	  $(call PrintConfCheckFailed)