Fix xdev-install when installing to a location other than /

Makefile.inc1

@@ -1743,9 +1743,10 @@ XDTP=/usr/${XDDIR}
 CDBENV=MAKEOBJDIRPREFIX=${MAKEOBJDIRPREFIX}/${XDDIR} \
 	INSTALL="sh ${.CURDIR}/tools/install.sh"
 CDENV= ${CDBENV} \
-	_SHLIBDIRPREFIX=${XDTP} \
-	TOOLS_PREFIX=${XDTP}
-CD2CFLAGS=-isystem ${XDTP}/usr/include -L${XDTP}/usr/lib -B${XDTP}/usr/lib
+	_SHLIBDIRPREFIX=${XDDESTDIR} \
+	TOOLS_PREFIX=${XDDESTDIR}
+CD2CFLAGS=-isystem ${XDDESTDIR}/usr/include -L${XDDESTDIR}/usr/lib \
+	-B${XDDESTDIR}/usr/lib
 CD2ENV=${CDENV} CC="${CC} ${CD2CFLAGS}" \
 	MACHINE=${XDEV} MACHINE_ARCH=${XDEV_ARCH}