add support of symbols export define in cpp interfaces

[bnb-ashershen]

Hello!

I've added basic support of define for cpp djinni interface symbols exporting. like was mentioned in #114

[artwyman]

Seems like a fine idea, but this repo isn't taking any new feature enhancements, per the note in the README:

Maintenance note: This repo is stable but no longer actively maintained. No further development is planned, and no new feature PRs will be merged. Bug fixes may be merged on a volunteer basis. If you have questions or want to talk to other users of Djinni, you can join the Slack community via the link at the end of this document.