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/cmExportBuildFileGenerator.h

@@ -19,6 +19,8 @@
 
 #include "cmExportFileGenerator.h"
 
+class cmExportCommand;
+
 /** \class cmExportBuildFileGenerator
  * \brief Generate a file exporting targets from a build tree.
  *
@@ -31,12 +33,17 @@
 class cmExportBuildFileGenerator: public cmExportFileGenerator
 {
 public:
+  cmExportBuildFileGenerator();
+
   /** Set the list of targets to export.  */
   void SetExports(std::vector<cmTarget*> const* exports)
     { this->Exports = exports; }
 
   /** Set whether to append generated code to the output file.  */
   void SetAppendMode(bool append) { this->AppendMode = append; }
+
+  /** Set the command instance through which errors should be reported.  */
+  void SetCommand(cmExportCommand* cmd) { this->ExportCommand = cmd; }
 protected:
   // Implement virtual methods from the superclass.
   virtual bool GenerateMainFile(std::ostream& os);
@@ -52,6 +59,7 @@ protected:
                                  ImportPropertyMap& properties);
 
   std::vector<cmTarget*> const* Exports;
+  cmExportCommand* ExportCommand;
 };
 
 #endif