ENH: Implemented link-interface specification feature.

- Shared libs and executables with exports may now have
    explicit transitive link dependencies specified
  - Created LINK_INTERFACE_LIBRARIES and related properties
  - Exported targets get the interface libraries as their
    IMPORTED_LINK_LIBRARIES property.
  - The export() and install(EXPORT) commands now give
    an error when a linked target is not included since
    the user can change the interface libraries instead
    of adding the target.

Source/cmExportFileGenerator.h

@@ -70,6 +70,11 @@ protected:
   void SetImportLinkProperties(const char* config,
                                std::string const& suffix, cmTarget* target,
                                ImportPropertyMap& properties);
+  void SetImportLinkProperties(const char* config,
+                               std::string const& suffix,
+                               cmTarget* target,
+                               std::vector<std::string> const& libs,
+                               ImportPropertyMap& properties);
 
   /** Each subclass knows how to generate its kind of export file.  */
   virtual bool GenerateMainFile(std::ostream& os) = 0;