bext/boost

The upstream boost repository we are using itself uses submodules (a LOT of
them).  For our fork to reference them, we'd also have to folk all (100+)
submodules in the BRL-CAD project.  In this case the main point is to have a
CMake build of boost someone else is maintaining, so we need to work around the
submodule issue.

In the BRL-CAD fork, the RELEASE branch we made has been modified to remove the
submodules and instead check in a set of fully populated clones from the
upstream boostorg/boost repository.  The procedure was roughly:

Prepare the upstream repository:

git clone https://github.com/boostorg/boost.git
mv boost boost_upstream
cd boost_upstream
git co boost-1.83.0
git submodule update --init --recursive --depth=1
For all the submodules (found by using "find . -name .git", excluding the top level .git folder)
rm -rf <path_to_submodule>/.git
cd ..


Check out the BRL-CAD fork (referenced the procedure at
https://gist.github.com/myusuf3/7f645819ded92bda6677 for part ofof this:)

git clone https://github.com/boostorg/boost.git
cd boost
git co boost-1.83.0
git switch -c RELEASE
git rm .gitmodules
For all the submodules (found by using "find . -name .git", excluding the top level .git folder)
git rm --cached <path_to_submodule>
git commit -m "Remove submodules"
cp -r ../boost_upstream/* .
git add -A
git commit -m "Populate repository with full copies of submodules"

For the next Boost release bump, in the release branch we'll merge the new tag,
prepare another copy of boostorg/boost.git the same way we did above, remove the
current files from the working copy of RELEASE (without committing), and copy in
the new boostorg/boost contents.  This will let us make a fully populated tree of
the next release.

Once the upstream RELEASE branch is ready, we bump the submodule reference in this repository
with:

git submodule update --remote boost (TODO - do we also need the --merge option here?)

Test the build.  If everything works:

git add boost
git commit -m "Update boost dependency to v<new version>"
git push