Add a tip about opt_VARS and whitespace.

Reviewed by:	wblock
Sponsored by:	Absolight
Differential Revision:	https://reviews.freebsd.org/D5447

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

@@ -4487,6 +4487,22 @@ BIN3_BUILD=	yes
 .else
 BIN3_BUILD=	no
 .endif</programlisting>
+
+	<tip>
+	  <para>Values containing whitespace must be enclosed in
+	    quotes:</para>
+
+	  <programlisting>OPT_VARS=	foo="bar baz"</programlisting>
+
+	  <para>This is due to the way &man.make.1; variable expansion
+	    deals with whitespace.  When <literal>OPT_VARS= foo=bar
+	      baz</literal> is expanded, the variable ends up
+	    containing two strings, <literal>foo=bar</literal> and
+	    <literal>baz</literal>.  But the submitter probably
+	    intended there to be only one string, <literal>foo=bar
+	      baz</literal>.  Quoting the value prevents whitespace
+	    from being used as a delimiter.</para>
+	</tip>
       </sect3>
 
       <sect3 xml:id="options-dependencies">