Optionally use OpenSSL or LibreSSL instead of bundled crypto-algorithms

crypto-algorithms "have no resistence to side-channel attacks and should not be used in contexts that need cryptographically secure implementations" (see lib/crypto-algorithms/README.md), so using OpenSSL or LibreSSL is preferable.

This does solve https://github.com/matrix-org/olm/issues/3 for some platforms, without breaking other platforms without these libraries (like web).

Signed-off-by: Lukas Lihotzki lukas@lihotzki.de