KNOWN_BUGS: 5.6 Improper use of Autoconf cache variables

Closes #603

docs/KNOWN_BUGS

@@ -42,6 +42,7 @@ problems may have been fixed or changed somewhat since this was written!
  5.3 libidn and old iconv
  5.4 AIX shared build with c-ares fails
  5.5 can't handle Unicode arguments in Windows
+ 5.6 Improper use of Autoconf cache variables
 
  6. Authentication
  6.1 NTLM authentication and unicode
@@ -246,6 +247,13 @@ problems may have been fixed or changed somewhat since this was written!
   https://curl.haxx.se/bug/?i=345
   https://curl.haxx.se/bug/?i=731
 
+5.6 Improper use of Autoconf cache variables
+
+ A number of macros used in curl's configure modify the Autoconf cache
+ variables which breaks configure variable caching.
+
+ Issue: https://github.com/curl/curl/issues/603
+
 
 6. Authentication