Add a note saying not to use := in *_DEPENDS variables.

Approved by:	wblock (mentor)
Sponsored by:	Absolight
Differential Revision:	https://reviews.freebsd.org/D3109

en_US.ISO8859-1/books/porters-handbook/makefiles/chapter.xml

@@ -3290,6 +3290,14 @@ ALWAYS_KEEP_DISTFILES=	yes
 		other:${PORTSDIR}/lang/other
 BUILD_DEPENDS=	${MY_DEPENDS}
 RUN_DEPENDS=	${MY_DEPENDS}</programlisting>
+
+	<important>
+	  <para><emphasis>Do not</emphasis> use <literal>:=</literal>
+	    to assign <varname>BUILD_DEPENDS</varname> to
+	    <varname>RUN_DEPENDS</varname> or vice-versa.  All
+	    variables are expanded immediately, which is exactly the
+	    wrong thing to do and almost always a failure.</para>
+	</important>
     </sect2>
 
     <sect2 xml:id="makefile-build_depends">