diff --git a/ChangeLog b/ChangeLog
index 4c0e3f83..95c024c7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2009-04-08  Ralf Wildenhues  <Ralf.Wildenhues@gmx.de>
+
+	Automake relies on the undocumented `_AC_COMPILER_EXEEXT' macro.
+	* lib/autoconf/lang.m4 (_AC_COMPILER_EXEEXT): Document that
+	Automake relies on this macro.
+
 2009-04-06  Eric Blake  <ebb9@byu.net>
 
 	Reduce testsuite size.
diff --git a/lib/autoconf/lang.m4 b/lib/autoconf/lang.m4
index 8bbe4945..bcd037be 100644
--- a/lib/autoconf/lang.m4
+++ b/lib/autoconf/lang.m4
@@ -626,6 +626,9 @@ AC_MSG_RESULT([$ac_cv_exeext])
 #
 # Must be run before _AC_COMPILER_OBJEXT because _AC_COMPILER_EXEEXT_DEFAULT
 # checks whether the compiler works.
+#
+# Do not rename this macro; Automake decides whether EXEEXT is used
+# by checking whether `_AC_COMPILER_EXEEXT' has been expanded.
 m4_define([_AC_COMPILER_EXEEXT],
 [AC_LANG_CONFTEST([AC_LANG_PROGRAM()])
 ac_clean_files_save=$ac_clean_files