Excluded/replaced/relocated ODB upstream files/directories:
  - libodb/odb/tr1/
  - libodb/odb/buildfile
  - libodb/odb/details/{condition.cxx lock.cxx mutex.cxx thread.cxx}
  - libodb/odb/details/posix/
  - libodb/odb/details/win32/* except for windows.hxx


  - libodb/odb/sqlite/buildfile
  - libodb/odb/sqlite/details/.gitignore
  - libodb/odb/sqlite/details/options.cli
  - libodb/odb/sqlite/details/pregenerated/ -- symlink options.?xx directly in details/

  - libodb/odb/version.hxx.in
  - libodb/odb/sqlite/version.hxx.in
  - libodb/odb/version.hxx        -- replace with pre-generated for submodule commit
  - libodb/odb/sqlite/version.hxx -- replace with pre-generated for submodule commit

SQLite is placed into a subdirectory to support the use of external version
(in which case we omit the corresponding -I option).

Submodule update procedure (similar to updating to a new version of a
third-party package):

1. Update submodule:

   https://build2.org/stage/build2-toolchain/doc/build2-toolchain-packaging.xhtml#core-version-management-new-version-submodule

2. Review upstream changes:

   https://build2.org/stage/build2-toolchain/doc/build2-toolchain-packaging.xhtml#core-version-management-new-version-review

   Look for:

   - files to add/remove (see above)
   - changes to buildfiles that would need to be replicated (including to
     root.build)

3. Replace verison.hxx with pre-generated for this submodule commit.

4. Review .gitattributes for any symlink=dir changes.