Based on functionality in dstat by Joerg Jung.

Remove unnecessary variable

Thanks to the FRIGN

Remove unnecessary cast to size_t

Function declarations should include variable names.
For functions with different parameters on different platfroms they are
left out.

size_t may not be enough, use 64 bit integers

Updated for style.

- Remove <errno.h> because related functions are in util.c now
 - Remove corresponding <string.h> if not used otherwise

This is (arguably) more elegant and it matches Xlib output logic.
Also use puts instead of printf as we don't do formating there.

I made a mistake while reading the Posix spec. In fact, the implicit
prerequisite imposed by the .c.o-rule is evaluated as expected, even if
the .c is not given in the target rule.

This partially reverts 0efd64ffaa.

This drops a lot of code and even though we are using a common function,
the specification of only the needed things keeps it very efficient and
even more efficient in memory and time than the previous solution.

There was a long tinkering process at farbfeld about this, but the sad
truth is that it's the only way to make the Makefile truly portable.
Listing it just as

   $(COM:=.o): config.mk $(REQ:=.h)

omits the dependency on the c-file itself, which incurs that strictly
speaking the object file is not depending on the source file, which is
nonsense.

You don't see strictly Posix compliant Makefiles around very often and
most use nasty GNU-extensions everywhere. It is a good idea to go ahead
as a fitting example and show how to write them portably.