Stop using BUILT_SOURCES

Commit dc7d2c49 introduced the use of BUILT_SOURCES to work around
missing dependencies. However, on closer reading of the gmake manual,
BUILT_SOURCES is mainly used in cases where the dependencies are not
know before the build starts (e.g. with auto-generated C header
files). Additionally, there are a number of drawbacks to
BUILT_SOURCES, which we already had to work around (e.g. in commit
eb732fb5).

Since we know all our dependencies statically, there's no need to use
this special variable. Let's rename it to GENERATED_FILES, which
doesn't have a special meaning to make, and add a few missing
dependencies (one of which was already broken by make -j on a clean
tree).

After this change, running make in a fully built tree is finally "clean":

  $ make
  make: Nothing to be done for `all'.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>