Export: Generate INTERFACE_LINK_LIBRARIES property on targets.

This property is generated only for targets which have recorded
policy CMP0022 as NEW, and a compatibility mode is added to
additionally export the old interfaces in that case too.

If the old interfaces are not exported, the generated export files
require CMake 2.8.12. Because the unit tests use a version which
is not yet called 2.8.12, temporarily require a lower version.

Source/cmExportFileGenerator.h

@@ -35,6 +35,8 @@ public:
   /** Set the namespace in which to place exported target names.  */
   void SetNamespace(const char* ns) { this->Namespace = ns; }
 
+  void SetExportOld(bool exportOld) { this->ExportOld = exportOld; }
+
   /** Add a configuration to be exported.  */
   void AddConfiguration(const char* config);
 
@@ -101,6 +103,10 @@ protected:
                                  cmGeneratorExpression::PreprocessContext,
                                  ImportPropertyMap &properties,
                                  std::vector<std::string> &missingTargets);
+  bool PopulateInterfaceLinkLibrariesProperty(cmTarget *target,
+                                 cmGeneratorExpression::PreprocessContext,
+                                 ImportPropertyMap &properties,
+                                 std::vector<std::string> &missingTargets);
   void PopulateInterfaceProperty(const char *propName, cmTarget *target,
                                  ImportPropertyMap &properties);
   void PopulateCompatibleInterfaceProperties(cmTarget *target,
@@ -128,9 +134,14 @@ protected:
                           std::vector<std::string> &missingTargets,
                           FreeTargetsReplace replace = NoReplaceFreeTargets);
 
+  void GenerateRequiredCMakeVersion(std::ostream& os,
+                                    const char *versionString);
+
   // The namespace in which the exports are placed in the generated file.
   std::string Namespace;
 
+  bool ExportOld;
+
   // The set of configurations to export.
   std::vector<std::string> Configurations;