bsd.obj.mk. Also, a make target called objwarn checks to see
if ${.OBJDIR} != ${.CURDIR} and ${.OBJDIR} != ${CANONICALOBJDIR}
and outputs a warning. (No warning for the latter if MAKEOBJDIR or MAKEOBJDIRP
REFIX is set). objwarn is called from all targets in bsd.prog.mk, bsd.kmod.mk,
and bsd.lib.mk.
Reviewed by: bde
b3618bcf57