-DMODVERSIONS isn't used anymore, so it can go. Also, after cleaning
up include/linux/module.h, it's obvious that we don't need
include/linux/modversions.h at all if CONFIG_MODVERSIONS not set,
no need to generate it.

Rules.make explicitly lists files which depend on modversions.h,
since make dep cannot know about the
"-include include/linux/modversions.h" which gets added to
the command line. Now that we understand when it is needed, we can
even get that list right ;-)

Oh well, nice theory. .hdepend will touch module.h when modversions.h
changes, so we still get unnecessary recompiles. We really need to
switch to the new way of dependency generation, it gets all that
right without even thinking about these special cases.

We don't track dependencies for .ver files. In fact, we relied on that
checksum would only change if the corresponding exporting C source
changes. That's not true, of course, all of the included headers have
say as well. So we better force the hash to be checked
unconditionally every time "make dep" is run.