[racket-dev] git submodule for native-library packages

Nothing has been split out of the current git repository, but there is
now a "native-pkgs" git submodule for the native-library packages.

If you build on Mac OS X or Windows or if you run a snapshot build,
then you'll need to use

  git pull
  git submodule init
  git submodule update

once on each repository checkout from now on, and

  git pull
  git submodule update

for updates after the first checkout.

If you just `git pull' without the submodule commands, then you'll have
an empty "native-pkgs" directory. That's fine if you're building on,
say, Linux. If you're on Mac OS X or Windows, then the makefile should
give you a good error message and suggest using `git submodule init'
and `git submodule update'.

If you use `git submodule update' today and forget to use it in the
future, probably things will be fine for a long time, because
"native-pkgs" doesn't change often. If "native-pkgs" does change and
you forget `git submodule update', then (as I understand things) `git
status' will tell you that there's a mismatch. The mismatch report
might take a form that's not entirely clear, but it should be enough to
remind you to run `git submodule update'.


I'm still unsure that submodules are going to be useful for managing a
kind of "main-distribution" repository with references to package
repositories. I'm much more confident that a submodule will work for
the native-library packages in the current build structure. The native
libraries are already kept in a separate repository, they change
infrequently, and almost no one has to change them. Also, the
"native-pkgs" git submodule replaces two things that I think were just
bad forms of `git submodule update':

 * A makefile target that fires on Mac OS X and uses `git pull', which
   is bad if you happen to be working offline.

 * A note in "INSTALL.txt" that tells Windows users to manually clone a
   particular git repository.

If a git submodule does not work out for native-library packages after
all, then should be easy to roll back, since there was no split to
rewind. Meanwhile, we get a little experience with git submodules.