Add a convenient way to add the includes install dir to the INTERFACE.

Export the INCLUDES DESTINATION without appending to the
INTERFACE_INCLUDE_DIRECTORIES of the target itself. That way, a target
can be exported multiple times with different INCLUDES DESTINATION
without unintended cross-pollution of export sets.

Source/cmExportFileGenerator.h

@@ -15,6 +15,8 @@
 #include "cmCommand.h"
 #include "cmGeneratorExpression.h"
 
+class cmTargetExport;
+
 /** \class cmExportFileGenerator
  * \brief Generate a file exporting targets from a build or install tree.
  *
@@ -114,7 +116,7 @@ protected:
   void GenerateInterfaceProperties(cmTarget *target, std::ostream& os,
                                    const ImportPropertyMap &properties);
   void PopulateIncludeDirectoriesInterface(
-                      cmTarget *target,
+                      cmTargetExport *target,
                       cmGeneratorExpression::PreprocessContext preprocessRule,
                       ImportPropertyMap &properties,
                       std::vector<std::string> &missingTargets);