curl: For SSL features, depend on thread-safety provided by our crypto plugins

curl: For SSL features, depend on thread-safety provided by our crypto plugins

To use SSL in curl, we need to initialize the SSL library in a thread-safe
manner and provide the appropriate callbacks. As we already do that in our
crypto plugins using these libraries, we depend on these features.

This implies that we need the same plugin enabled (openssl, gcrypt) as the
curl backend is configured to use to fetch from HTTPS URIs.