bsd.port.subdir.mk, without the overhead of an additional ports tree.
Use
make BSDPORTMK='${PORTSDIR}/My.Mk/bsd.port.mk' target
and
make BSDPORTSUBDIRMK='${PORTSDIR}/My.Mk/bsd.port.subdir.mk' target
to build with the alternate versions.
MFC after: 3 days