Because not all compilers under test support the existing 'activation
features', add another test to ensure that compilers which support
both static_assert and nullptr behave as expected in this test.
The tests below test the presence of both cxx_final and cxx_override,
only one of which is supported by MSVC. The test is in part intended
to verify that the COMPILE_FEATURES genex supports multiple arguments
and allows users to define names for groups of features (Clang already
calls cxx_final and cxx_override 'override control' as a group). Keep
the test, and allow the expectation to be set as appropriate.
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.
Stephen Kelly