Add a blurb on how GNOME auto-detection works using the WANT_*/HAVE_*

macros.

en/gnome/docs/porting.sgml

@@ -70,6 +70,32 @@ WANT_GNOME=	yes
 	      usually used for backend APIs.</p></li>
           </ul>
 
+		  <p>If your port can optionally use GNOME, you must set 
+		  <tt>WANT_GNOME= yes</tt> in your Makefile, then check to see if
+		  <tt>HAVE_GNOME</tt> is set.  Since this is a conditional
+		  evaluation, you need to stick it between <tt>bsd.port.pre.mk</tt>
+		  and <tt>bsd.port.post.mk</tt>.  For example:</p>
+
+		  <pre>
+WANT_GNOME=	yes
+
+.include &lt;bsd.port.pre.mk&gt;
+
+.if defined(HAVE_GNOME)
+USE_GNOME=	yes
+CONFIGURE_ARGS+=	--with-gnome
+.else
+CONFIGURE_ARGS+=	--without-gnome
+.endif
+
+.include &lt;bsd.port.post.mk&gt;
+          </pre>
+
+		  <p>What happens here is <tt>WANT_GNOME</tt> tells the ports system
+		  to check for the existence of <tt>gnome-config</tt>.  If it 
+		  exists, <tt>HAVE_GNOME</tt> is set.  If not, <tt>HAVE_GNOME</tt>
+		  remains unset.</p>
+
           <p>When building GNOME ports, remember that many applications 
 		  require shared directories in <tt>${PREFIX}/share/gnome</tt>.  Ports
 		  should be constructed in a way such that files placed in these