ports.7: Add an example of setting variables for selected ports

Reviewed by:	crees
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D28202

share/man/man7/ports.7

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd April 6, 2021
+.Dd April 17, 2021
 .Dt PORTS 7
 .Os
 .Sh NAME
@@ -645,6 +645,18 @@ shells_zsh_UNSET=	DOCS EXAMPLES
 .Pp
 These and other options-related variables are documented in
 .Pa /usr/ports/Mk/bsd.options.mk .
+.It Sy Example 5\&: No Setting Xr make 1 Variables for Specific Ports via Xr make.conf 5
+.Pp
+The following example shows how to set arbitrary
+.Xr make 1
+variables only specific ports:
+.Bd -literal -offset 2n
+# Set DISABLE_MAKE_JOBS for the lang/rust port:
+\&.if ${.CURDIR:M*/lang/rust}
+DISABLE_MAKE_JOBS=	yes
+TRYBROKEN=		yes
+\&.endif
+.Ed
 .El
 .Sh SEE ALSO
 .Xr make 1 ,