[ Full content available at: https://github.com/apache/trafficserver/pull/4140 ]
This enables conditional compilation checks in pure C++ code, without support from `configure.ac` or `#define`. It is a bit more verbose but has the advantage of being self contained - the code does not depend on external tools in order to compile correctly. It is also more robust because it does its checks with the exact compiler and library environment. Therefore changing the environment and recompiling will change the output without other rebuilding. An example of its use in a real life situation [is here](https://github.com/apache/trafficserver/pull/4040#discussion_r211333263) in #4040. This creates a wrapper function for an `openSSL` call that compiles to the underlying call if present, and nothing if not.
