source.mak: Don't manually list object files

Rather than manually listing everything in `OBJECTS`, use portable variable substitution to expand it from `SOURCES`.  Tested with GNU and BSD makes -- and I double-checked with `diff` that `$(OBJECTS)` expanded the same before and after.

I didn't touch *mk_mpw.mak* that duplicates the list of sources because I have no idea why it does than nor know MPW, but it probably could benefit either from including *source.mak* or having a similar fix than the one here.