Fix for OpenSSL 1.1.1 on boost/asio

[cpsauer]

Thanks for building such a great library!

I was patching things to work on with OpenSSL 1.1.1 locally, which seemed like a good idea, since that's the latest LTS version. It turned out it was super easy, so I wanted to try to contribute the change back in case that's helpful.

All you need to do to get things to build with OpenSSL 1.1.1--at least for MacOS and iPhoneOS--is to change X509_STORE_CTX_get_chain to X509_STORE_CTX_get0_chain in x509_cert_utilities.cpp. Upgrading to the latest version of boost/asio and websocketpp as fixes all the other issues. It looks like they just renamed X509_STORE_CTX_get0_chain, since internally, the compatibility flags just redefine X509_STORE_CTX_get_chain to X509_STORE_CTX_get0_chain.

If you want to maintain compatibility with openssl 1.0, just conditionally compile around OPENSSL_VERSION like (in x509_cert_utilities.cpp):

#if (OPENSSL_VERSION_NUMBER < 0x10100000L)
    STACK_OF(X509)* certStack = X509_STORE_CTX_get_chain(storeContext);
#else 
    STACK_OF(X509)* certStack = X509_STORE_CTX_get0_chain(storeContext);
#endif

[BillyONeal]

Care to PR-ize this?

[cpsauer]

Totally. Just wanted to check in first. See PR, above.