Problem with bsd.ports.mk

[walt]

My /usr/ports and /usr/dfports are symlinks to another disk mounted
on /opt.  When I build a DFly port override I get this warning:

/opt/dfports/devel/libtool15 is not a subdirectory of /usr/ports
"/usr/share/mk/bsd.port.mk", line 6: warning: "/usr/bin/relpath /usr/ports
/opt/dfports/devel/libtool15" returned non-zero status

This warning doesn't break the build, but it does break portupgrade
which stops with a 'broken makefile' error.