CMake feature detection

[rleigh-codelibre]

As mentioned in https://lists.boost.org/Archives/boost/2018/10/243729.php I would be happy to implement this type of functionality.

Would a PR implementing the detection logic be acceptable? Or is someone already working on this?

Thanks, Roger

[jzmaddock]

I'm quite certain that I'm not qualified to comment - anyone else?

[eldiener]

I am not qualified to judge a CMake implementation either. I did suggest that doing this for config ( and predef also ) and then having current Boost libraries which are converted to CMake use the CMake implementations in config and predef to replace the bjam code they already have now is a much better solution than having each library do their own CMake implementation for what they will need.

[rleigh-codelibre]

I have done some initial work on this in the linked PR (#249). There will be more to come, this is just the initial step to make it installable and re-usable by downstream consumers.