Build BitShares-Core Linux binaries with Ubuntu 18.04 LTS (Bionic Beaver)

[abitmore]

Build BitShares-Core Linux binaries with Ubuntu 18.04 LTS (Bionic Beaver) and bump versions of OpenSSL and curl libraries.

Fixes #49.

[abitmore]

test-6.0.0 fails to build with OSX cross-build:

Undefined symbols for architecture x86_64:
  "_CFRelease", referenced from:
      _Curl_resolv in libcurl.a(libcurl_la-hostip.o)
  "_SCDynamicStoreCopyProxies", referenced from:
      _Curl_resolv in libcurl.a(libcurl_la-hostip.o)
ld: symbol(s) not found for architecture x86_64

Related code in curl: https://github.com/curl/curl/blob/curl-7_79_1/lib/hostip.c#L667-L669

#if defined(ENABLE_IPV6) && defined(CURL_OSX_CALL_COPYPROXIES)
    {
      /*
       * The automagic conversion from IPv4 literals to IPv6 literals only
       * works if the SCDynamicStoreCopyProxies system function gets called
       * first. As Curl currently doesn't support system-wide HTTP proxies, we
       * therefore don't use any value this function might return.
       *
       * This function is only available on a macOS and is not needed for
       * IPv4-only builds, hence the conditions above.
       */
      CFDictionaryRef dict = SCDynamicStoreCopyProxies(NULL);
      if(dict)
        CFRelease(dict);
    }
#endif

[abitmore]

The error was caused by a recent change (and follow-ups) in curl which meant to support "NAT64 address synthesis" in macOS, introduced since curl 7.77.0: curl/curl@31f631a

It made the "SystemConfiguration framework" a dependency for curl mac build. We do have the framework in SDK, so it compiles fine. But we didn't link with it. Since we only link libraries statically, I'm not sure what's the best way to link with the framework.

The new code exists only when IPv6 support is enabled when building curl (which is enabled by default). If we disable IPv6 support in macOS build, we do not need to link with the framework.

Another option is to revert to an older version of curl.

[abitmore]

Decided to build macOS binaries with an older curl version (7.76.1).