Add bundled bdwgc option

[ivmai]

No description provided.

[kou]

Thanks!

But we don't want to bundle bdwgc. We want to use system bdwgc instead. If system bdwgc is enough recent, we'll use bdwgc. Otherwise, we'll use bundled libgcroots. We'll remove bundled libgcroots eventually.

Can I push to this branch something? I'll add a CI job that install unreleased bdwgc separately and use it as a system bdwgc.

[ivmai]

We want to use system bdwgc instead.

Agree.

Can I push to this branch something? I'll add a CI job that install unreleased bdwgc separately and use it as a system bdwgc.

Sure.