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>