Mention the meaning of CYGWIN.

diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index c33938aac717d7f610873a729a5d3cc16e9ad705..ae70b45b90c4384e80330859c588dc05e854ca48 100644 (file)
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -8879,6 +8879,10 @@ case $host_os in
          * ) CYGWIN=no;;
 esac
 @end example
+
+Beware that the variable @code{CYGWIN} has a very special meaning when
+running CygWin32, and should not be changed.  That's yet another reason
+not to use this macro.
 @end defmac
 
 @defmac AC_DECL_YYTEXT