add a consistent set of text about X11

2011/toledo/index.html

@@ -408,8 +408,10 @@ Location: <a href="../../location.html#MX">MX</a> - <em>United Mexican States</e
 
 <!-- BEFORE: 1st line of markdown file: 2011/toledo/README.md -->
 <h2 id="to-build">To build:</h2>
-<p>If you have X11 installed:</p>
 <pre><code>    make</code></pre>
+<p>NOTE: this entry requires the <code>X11/Xlib.h</code> header file and the X11 library to
+compile. For more information see the
+FAQ on “<a href="../../faq.html#X11">X11</a>”.</p>
 <p>There are two alternate versions, <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2011/toledo/toledo.alt.c">one that lets you reconfigure the
 controls and size</a> and <a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2011/toledo/toledo.alt2.c">another that should work for
 Windows</a>. See <a href="#alternate-code">Alternate code</a> below.</p>