diff --git a/doc/build_system.txt b/doc/build_system.txt
index 20242b1353..1efe6b5399 100644
--- a/doc/build_system.txt
+++ b/doc/build_system.txt
@@ -16,6 +16,9 @@ DESTDIR
     Destination directory for the install targets, useful to prepare packages
     or install FFmpeg in cross-environments.
 
+GEN
+    Set to ‘1’ to generate the missing or mismatched references.
+
 Makefile targets:
 
 all