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.

Source/cmGeneratorExpression.h

@@ -117,6 +117,9 @@ public:
     this->EvaluateForBuildsystem = eval;
   }
 
+  void GetMaxLanguageStandard(cmTarget const* tgt,
+                    std::map<std::string, std::string>& mapping);
+
 private:
   cmCompiledGeneratorExpression(cmListFileBacktrace const& backtrace,
               const std::string& input);
@@ -134,6 +137,8 @@ private:
   mutable std::set<cmTarget*> DependTargets;
   mutable std::set<cmTarget const*> AllTargetsSeen;
   mutable std::set<std::string> SeenTargetProperties;
+  mutable std::map<cmTarget const*, std::map<std::string, std::string> >
+                                                          MaxLanguageStandard;
   mutable std::string Output;
   mutable bool HadContextSensitiveCondition;
   bool EvaluateForBuildsystem;