add a consistent set of text about X11

1998/banks/README.md

@@ -4,6 +4,10 @@
     make all
 ```
 
+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)".
+
 If you do not have a page up or page down key you might wish to see the
 [Alternate code](#alternate-code) section below. You may redefine all the keys
 and the time step with predefined macros, however, so one lacking page up / page

1998/banks/index.html

@@ -409,6 +409,9 @@ Location: <a href="../../location.html#US">US</a> - <em>United States of America
 <!-- BEFORE: 1st line of markdown file: 1998/banks/README.md -->
 <h2 id="to-build">To build:</h2>
 <pre><code>    make all</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>If you do not have a page up or page down key you might wish to see the
 <a href="#alternate-code">Alternate code</a> section below. You may redefine all the keys
 and the time step with predefined macros, however, so one lacking page up / page