Excluded/replaced/relocated upstream files/directories:
  - libpkg-config/libbpkg-config/{.gitignore, buildfile}

  - libpkg-config/libbpkg-config/version.h.in
  - libpkg-config/libbpkg-config/version.h -- replace with pre-generated for
                                              submodule commit

  - libpkg-config/libbpkg-config/config.h.in
  - libpkg-config/libbpkg-config/config.h -- replace with pre-generated with
                                             empty system/default path (we
					     do/should not use them).

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.h, config.h with pre-generated for this submodule commit.