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.
Revert "Add the WriteCompilerDetectionHeader module."

This reverts commit 50d43cf5555f98c3811bd7897eb43bc27bf8d215.
Merge topic 'WriteCompilerDetectionHeader-module' into next

f9e85b9a Revert "Add the WriteCompilerDetectionHeader module."
Merge topic 'CMAKE_CXX_KNOWN_FEATURES-global-property' into next

feca7408 Features: Make CMAKE_CXX_KNOWN_FEATURES a property.