Making dependencies once up front is not ideal. For one, you don't need
them initially, since when you don't have the .o file, you bet you have
to build it no matter what the dependencies say - dependencies are about
deciding when to *re*build.

There's more reasons, like:
o you don't even know which files you'll build, so you have to go over
  all files (even over drivers/{sbus,s390,...} on i386)
o generated files don't exist yet, so you cannot pick up dependencies
  on them
o even if dependencies are right initially, they change when you work on 
  your tree or patch it, and nobody will notice unless you run "make dep"
  explicitly again

Anyway, gcc knows hows to emit a correct dependency list, so we just use
that. Well, a little bit of hacking is necessary to remove the dependency
on autoconf.h and put in individual CONFIG_WHAT_EVER dependencies instead,
since otherwise changing one config option would cause everything to be
rebuilt.

I should add that I didn't come up with this all by myself, most work
is actually done in gcc and there were discussions about using -MD on
kbuild-devel way back, so I should mention Keith Owens and Michael
Elizabeth Chastain, and probably others that I forgot, so I apologize
just in case.