* configure.ac: Tweak the "unported" error message.

"Has not been ported to" usually isn't quite correct these days.
Eg it could be an old platform that we dropped support for.

ChangeLog

+2014-03-05  Glenn Morris  <rgm@gnu.org>
+
+	* configure.ac: Tweak the "unported" error message.
+
 2014-03-05  Paul Eggert  <eggert@cs.ucla.edu>
 
 	Fix configuration bug on Solaris 2.5.1 (Bug#16905).

configure.ac

@@ -653,7 +653,8 @@ fi
 dnl quotation ends
 
 if test $unported = yes; then
-  AC_MSG_ERROR([Emacs hasn't been ported to `${canonical}' systems.
+  AC_MSG_ERROR([Emacs does not support building on `${canonical}' systems.
+If you think it should, please send a report to bug-gnu-emacs@gnu.org.
 Check `etc/MACHINES' for recognized configuration names.])
 fi