Update makefiles to prevent re-invocation of make

The makefiles used to read in makefiles of automatically generated prerequisites
(.d files) and supply rules to update them.

After reading in all makefiles, make will consider each as a goal target and
attempt to update it. After all makefiles have been checked, if any have
actually been changed, make starts with a clean slate and reads all the
makefiles over again (From section 3.5 'How Makefiles Are Remade' of the GNU
Make manual).

The latter is unnecessary for makefiles of prerequisites as used by this
project: since we know some prerequisite of the target changed we know that the
respective object file has to be rebuilt. Having the updated prerequisites won't
affect that decision. Therefore, building the makefile of prerequisites along
with its object file is more efficient. For the next build the updated
prerequisites will be available.

See <http://mad-scientist.net/make/autodep.html> where this method is described
by Paul Smith (section heading: 'Avoiding Re-exec of make'). I made a minor
change by adding rules with no prerequisites or recipe for all .d files, only
using the include directive for existant makefiles and adding each .d file to
the prerequisites of its object file.

This way, if a .d file doesn't exist, the rule for its object file will always
run: make will consider the non-existant .d file out of date and imagine it to
have been updated despite its own rule lacking a recipe. As usual the .d will
really be created along with the object file.

As we don't try to include non-existant .d files, re-invocation of make will be
prevented in that case also. Concerning already existant object files I expect
my changes to make no difference, since make will always consider them up to
date.

Also do some cosmetic changes to the makefiles.