diff --git a/docs/tools/Makefile.am b/docs/tools/Makefile.am
index 31982aec39..9287540464 100644
--- a/docs/tools/Makefile.am
+++ b/docs/tools/Makefile.am
@@ -1,3 +1,5 @@
+AUTOMAKE_OPTIONS = subdir-objects
+
 include $(top_srcdir)/Makefile.decl
 
 AM_CPPFLAGS = \