bsd.ports.mk sets STRIP= after all Uses files have been processed initially.
If you have WITH_DEBUG=yes set, cmake's INSTALL_TARGET has been set already to
install/strip, before STRIP is reset.

The quick fix also evaluates WITH_DEBUG, which should be checked anyways,
since this is the correct user-settable argument, that should have precedence
over implementation details of bsd.port.mk.

Test:

/usr/ports/graphics/openshadinglanguage # make -VWITH_DEBUG -VINSTALL_TARGET
yes
install/strip