Provide default for $(TREE)

If the $(TREE) variable is not defined (which could be the case for smaller
projects - used to test features coming soon... ;-)), it defaults to the CWD
which sounds reasonable for projects which don't bother to declare $(TREE).
Support automatic prerequisites generation

For .S and .c files, automatic prerequisites generation by means of gcc -M
is now supported. The dependency files .X.deps where X is the name of the
original file, are automatically generated and updated.
Merge branch 'master' of https://github.com/tboege/Makefile.build

Conflicts:
	Makefile.build

This brought the TODO file into my local copy (whew, it must have been
really out of date) and I eliminated the entry about automatic prerequisites
generation there.