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.
Merge topic 'COMPILE_FEATURES-genex' into next

0dfe395e Features: Add COMPILE_FEATURES generator expression.