The variables are not meant to be defined there really, but only with
the third-party project.
Using them, e.g. with the following, would actually result in libvterm
not being found then:
make CMAKE_EXTRA_FLAGS="-DUSE_BUNDLED_LIBVTERM=ON" \
DEPS_CMAKE_FLAGS="-DUSE_BUNDLED=OFF -DUSE_BUNDLED_LIBVTERM=ON"
In https://github.com/neovim/neovim/pull/6357 they were renamed to
`USE_BUNDLED_X` from `X_USE_BUNDLED`, but the above reasoning applies
to the old names, too.
Internally `CMAKE_PREFIX_PATH` is used to add the built/bundled third
party packages for `find_package`, so there is no reason to e.g. query
the values via `load_cache` for example from the third-party project.
