CDRIVER-2083 Update .cmake variable naming scheme

This commit updates the libbson submodule commit point.

libbson already publishes two .cmake files, one for shared builds and one
for static builds.  The updated libbson commit publishes variables under
separate namespaces, BSON_* and BSON_STATIC_*.

This commit uses those .cmake files directly instead of our own FindBSON
config file and uses the new variable names where appropriate.

This commit also changes the generated .cmake files for libmongoc to
follow the same MONGOC_* and MONGOC_STATIC_* naming convention for
downstream users.

mongocxx (and maybe the PHP driver) will need a corresponding update.