[wwwdocs,fortran] Update link to CHKSYS

[Gerald Pfeifer]

This one has been failing for quite a while, and I found
  http://flibs.sourceforge.net/chksys.html
as a potential replacement link.

Thoughts?

Gerald

Index: readings.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/readings.html,v
retrieving revision 1.276
diff -u -r1.276 readings.html
--- readings.html	11 Apr 2017 22:30:59 -0000	1.276
+++ readings.html	11 Apr 2017 22:39:10 -0000
@@ -436,7 +436,7 @@
         possible to stress the compiler error handling.
       </li>
       <li>
-        <a href="http://ftp.cac.psu.edu/pub/ger/fortran/chksys.htm"><cite>Checking
+        <a href="http://flibs.sourceforge.net/chksys.html"><cite>Checking
         properties of the compiler and the run-time environment</cite></a> by
         Arjen Markus (source provided).
       </li>