SSL: fixed build with LibreSSL 2.8.0 (ticket #1605). LibreSSL 2.8.0 "added const annotations to many existing APIs from OpenSSL, making interoperability easier for downstream applications". This includes the const change in the SSL_CTX_sess_set_get_cb() callback function (see 9dd43f4ef67e), which breaks compilation. To fix this, added a condition on how we redefine OPENSSL_VERSION_NUMBER when working with LibreSSL (see 382fc7069e3a). With LibreSSL 2.8.0, we now set OPENSSL_VERSION_NUMBER to 0x1010000fL (OpenSSL 1.1.0), so the appropriate conditions in the code will use "const" as it happens with OpenSSL 1.1.0 and later versions.

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