This looks like a good use case for submodules, because the
native-library packages change infrequently, and no one cares
about the history of changes relative to the rest of the
project (except to be able to get a set of packages that is
consistent with the rest of the repository).
The change avoids the problem that `make' on Mac OS X would
try a `git update', which is no good if you happen to be
offline (and have your "buidl.native-pkgs" repo pull from the
obvious place).
Finally, it's easier for Windows users, since `git submodule init'
and `git submodule update' is easier to use and remember than
`git clone <some-repository-URL>'. The makefile more helpfully
complains if "native-pkgs" doesn't seem to have been initialized
as a submodule.
