Fix export file variable pollution

Add missing unset of scratch variables to generated export file for case
that the file was already included. We already unset these when parsing
the file normally, but the multiple inclusion case was leaving them
around.

Source/cmExportFileGenerator.cxx

@@ -1040,6 +1040,9 @@ void cmExportFileGenerator::GenerateExpectedTargetsCode(std::ostream& os,
         "  endif()\n"
         "endforeach()\n"
         "if(\"${_targetsDefined}\" STREQUAL \"${_expectedTargets}\")\n"
+        "  unset(_targetsDefined)\n"
+        "  unset(_targetsNotDefined)\n"
+        "  unset(_expectedTargets)\n"
         "  set(CMAKE_IMPORT_FILE_VERSION)\n"
         "  cmake_policy(POP)\n"
         "  return()\n"