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/cmExportCommand.h

@@ -19,6 +19,8 @@
 
 #include "cmCommand.h"
 
+class cmExportBuildFileGenerator;
+
 /** \class cmExportLibraryDependenciesCommand
  * \brief Add a test to the lists of tests to run.
  *
@@ -93,6 +95,9 @@ private:
   cmCAEnabler Append;
   cmCAString Namespace;
   cmCAString Filename;
+
+  friend class cmExportBuildFileGenerator;
+  std::string ErrorMessage;
 };