Silence a warning about REALM_VERSION_MAJOR being undefined when building realm-js with core v2.8.x.
Silence a warning about REALM_VERSION_MAJOR being undefined when
building realm-js with core v2.8.x.

Core v3.0 renamed `REALM_VER_MAJOR` to `REALM_VERSION_MAJOR`. If the
latter isn't defined, we'll #define it to the former so that our version
checks see the expected core version value.

realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-object-store,realm/realm-object-store,realm/realm-object-store