bsd.python.mk implements the USE_PYTHON, USE_PYTHON_BUILD and USE_PYTHON_RUN
switches. The two latter only trigger runtime/buildtime dependencies, while
USE_PYTHON entails both.
bsd.port.mk however only checks for USE_PYTHON and does not include bsd.python.mk
if USE_PYTHON_BUILD or USE_PYTHON_RUN are set, thus making those switches effec-
tively unavailable to potential users.
Over to maintainers.