Fix feature tests.
Merge topic 'CMAKE_CXX_KNOWN_FEATURES-global-property' into next

e7c0ef11 Fix feature tests.
Features: Make CMAKE_CXX_KNOWN_FEATURES a property.

As a 'built-in' variable it imposes a cost on all variable lookups
and it is expected to be rarely used.
Merge topic 'CMAKE_CXX_KNOWN_FEATURES-global-property' into next

3fdfa5d3 Features: Make CMAKE_CXX_KNOWN_FEATURES a property.