place object file names each on a new line in Makefiles

That is done to avoid conflicts.
Often two parallel branches add new files to the build, so they have to
modify the same line in a Makefile. That causes a merge conflict.
This commit makes file names appear each on a new line, so the chance of
a conflict is reduced.
Merge pull request #37 from natsys/makefile-split-file-names

place object file names each on a new line in Makefiles