SSL: let it build against BoringSSL.

This change adds support for using BoringSSL as a drop-in replacement
for OpenSSL without adding support for any of the BoringSSL-specific
features.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>
SSL: let it build against LibreSSL.

LibreSSL developers decided that LibreSSL is OpenSSL-2.0.0, so tests
for OpenSSL-1.0.2+ are now passing, even though the library doesn't
provide functions that are expected from that version of OpenSSL.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>