CDRIVER-1337 introduced a BSON_EXPERIMENTAL_FEATURES and MONGOC_EXPERIMENTAL_FEATURES flags for experimental 3.4 features in libbson and libmongoc 1.4.0, respectively.

Presently, our Decimal128 implementation is the only code that utilizes these features. For the upgrade to 1.4.0, we should wrap the Decimal128-specific code in an #ifdef BSON_EXPERIMENTAL_FEATURES block.

We can disable BSON_EXPERIMENTAL_FEATURES for the bundled libbson and libmongoc build and defer to the system library otherwise. In that case, the class will be available if the system library was compiled to support experimental features.