LEGAL: add legal situation of aclocal.m4 [ci skip]

FSF holds its copyright.

LEGAL

@@ -192,6 +192,17 @@ configure::
     This configure script is free software; the Free Software Foundation
     gives unlimited permission to copy, distribute and modify it.
 
+aclocal.m4::
+
+  This file is free software.
+
+  >>>
+    Copyright (C) 1996-2020 Free Software Foundation, Inc.
+
+    This file is free software; the Free Software Foundation
+    gives unlimited permission to copy and/or distribute it,
+    with or without modifications, as long as this notice is preserved.
+
 tool/config.guess::
 tool/config.sub::