Features: FATAL_ERROR on compilers with no recorded features.

Users of the new target_compile_features command are expected to
check the existence of the CMAKE_CXX_COMPILE_FEATURES variable before
attempting to use it to require features.
Tests: Change target_compile_features test execution logic.

Run the tests if there are known features for the compiler - force the
tests to be kept up-to-date with the compilers for which any features
are recorded.
Merge topic 'feature-absence-hard-error' into next

b8dd2f44 Tests: Change target_compile_features test execution logic.
44957b4b Features: FATAL_ERROR on compilers with no recorded features.
Add missing files.
Merge topic 'feature-absence-hard-error' into next

112a9312 Add missing files.