Tell dependencies if vendored OpenSSL was used

The system OpenSSL knows where its certificates are. If
DEP_OPENSSL_VENDORED is not set:
- openssl-probe doesn't need to set any environment variables and can
get the paths from OpenSSL itself.
- Libraries that normally use `openssl_probe::probe()` and
`SSL_CTX_load_verify_locations` can instead use
`SSL_CTX_set_default_verify_paths`.