This was added in OpenSSL 1.1.0. cryptography.io binds it. They don't
actually use it, but this is a useful feature to have anyway. Projects
like Envoy currently implement such a mode with
X509_STORE_set_verify_cb, which is a very problematic API to support.
Add this so we can move them to something more sustainable.
Change-Id: Iaff2d08daa743e0b5f4be261cb785fdcd26a8281
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/53965
Commit-Queue: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>