We require that the $<$<COMPILE_FEATURES:cxx_feature>:empty2> generates
a '1' when CXX11 is enabled. GNU 4.4 does not support cxx_nullptr, but
does support cxx_auto_type.
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