Allow DEPENDOBJS/DEPENDSRCS to work with only OBJS set and no SRCS.

Allow DEPENDOBJS/DEPENDSRCS to work with only OBJS set and no SRCS.

Default to tracking .depend.* for OBJS rather than SRCS.

This helps cover some special case builds like gnu/lib/csu which
do more of a PROGS-like thing with bsd.prog.mk.

It is possible this causes out-of-tree Makefiles to have problems if they use
this pattern:
	foo.o: foo.c
		${CC} -o ${.TARGET} ${.ALLSRC}
This may cause multiple source files to be compiled due to finding the
'foo.o: foo.c' dependency both in the Makefile at the .depend file. Or
it may try compiling headers. This can be worked around by either of these:
	foo.o: foo.c
		${CC} -o ${.TARGET} ${.ALLSRC:N*.h:[1]}
Or
	foo.o: foo.c
		${CC} -o ${.TARGET} ${.CURDIR}/foo.c
In the latter case the ${.CURDIR} may need to be a different path. The
first case covers automatically using .PATH.

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