add a consistent set of text about X11

2015/dogon/README.md

@@ -4,6 +4,10 @@
     make
 ```
 
+NOTE: this entry requires the `X11/Xlib.h` header file and the X11 library to
+compile.  For more information see the
+FAQ on "[X11](../../faq.html#X11)".
+
 To change the dimensions you can use the `WIDTH` and `HEIGHT` variables when
 compiling (default 1024 x 800):
 

2015/dogon/index.html

@@ -409,6 +409,9 @@ Location: <a href="../../location.html#IL">IL</a> - <em>State of Israel</em> (<e
 <!-- BEFORE: 1st line of markdown file: 2015/dogon/README.md -->
 <h2 id="to-build">To build:</h2>
 <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>To change the dimensions you can use the <code>WIDTH</code> and <code>HEIGHT</code> variables when
 compiling (default 1024 x 800):</p>
 <pre><code>    make clobber WIDTH=640 HEIGHT=480</code></pre>