OpenSSL Engine causes compilation issues on iOS

[johnhe4]

This seems related to #713 and is not addressed by #753. Realized on tag v0.13.0

Applications targeting iOS, visionOS, and other mobile platforms may include a compiled version of OpenSSL libraries in order to support other TLS-enabled libraries, including rabbitmq-c. When compiling OpenSSL for such platforms, the 1.1.1x OpenSSL configuration explicitly disables the ENGINE interface, as seen in this base config snippet from the 15-ios.conf file:

"ios-common" => {
        template         => 1,
        inherit_from     => [ "darwin-common" ],
        sys_id           => "iOS",
        disable          => [ "engine", "async" ],
    },

This make sense considering such devices have a distribution system that strongly discourages (and even disallows altogether) dynamic linking.

The rabbitmq-c library has a dependency on the ENGINE interface and will not compile without it, but in terms of functionality it seems that rabbitmq-c as a library shouldn't require the ENGINE interface.

Would it make sense to have a cached CMake flag to disable the Engine dependency? What other ways can we work around this for mobile platforms?