Features: Add COMPILE_FEATURES generator expression.
Allow setting build properties based on the features available for a target. The availability of features is determined at generate-time by evaluating the link implementation. Ensure that the <LANG>_STANDARD determined while evaluating COMPILE_FEATURES in the link implementation is not lower than that provided by the INTERFACE of the link implementation. This is similar to handling of transitive properties such as POSITION_INDEPENDENT_CODE.