Makefile: list generated object files in OBJECTS

Makefile: list generated object files in OBJECTS

Set the OBJECTS variable to a comprehensive list of all object
file targets.  To make sure it is truly comprehensive, restrict
the scope of the %.o pattern rule to only generate objects in
this list.

Attempts to build other object files will fail loudly:

	$ touch foo.c
	$ make foo.o
	make: *** No rule to make target `foo.o'.  Stop.

providing a reminder to add the new object to the OBJECTS list.

The new variable is otherwise unused.  The intent is for later
patches to take advantage of it.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>