[CMake] empty CMAKE_CXX_KNOWN_FEATURES in cmake 3.1.0-rc2

[Stephen Kelly]

Erik Sjölund wrote:

> CMAKE_CXX_KNOWN_FEATURES seems to be empty in
> cmake 3.1.0-rc2
> 
> Should it be?

Yes, it's not a variable, but a global property:

 get_property(cxx_features GLOBAL PROPERTY CMAKE_CXX_KNOWN_FEATURES)
 message("known: ${cxx_features}")

Thanks,

Steve.