CGAL:: Add Parallel_if_available_tag

STL_Extension/include/CGAL/tags.h

@@ -58,6 +58,13 @@ struct Null_functor {
 struct Sequential_tag {};
 struct Parallel_tag : public Sequential_tag {};
 
+#ifdef CGAL_LINKED_WITH_TBB
+typedef CGAL::Parallel_tag Parallel_if_available_tag;
+#else
+typedef CGAL::Sequential_tag Parallel_if_available_tag;
+#endif
+  
+
 // A function that asserts a specific compile time tag
 // forcing its two arguments to have equal type.
 template <class Base>