diff --git a/gtk-doc.make b/gtk-doc.make
index 264a7cb..d0d62e6 100644
--- a/gtk-doc.make
+++ b/gtk-doc.make
@@ -23,7 +23,9 @@ GPATH = $(srcdir)
 
 TARGET_DIR=$(HTML_DIR)/$(DOC_MODULE)
 
-EXTRA_DIST = 				\
+# Initialize EXTRA_DIST if not already set.
+EXTRA_DIST ?=
+EXTRA_DIST += 				\
 	$(content_files)		\
 	$(HTML_IMAGES)			\
 	$(DOC_MAIN_SGML_FILE)		\
