CGAL:: Add Parallel_if_available_tag

STL_Extension/include/CGAL/tags.h

@@ -26,7 +26,7 @@ namespace CGAL {
 struct Void {};
 
 // Boolean_tag<bool> is a model of the Boost Integral Constant concept.
-//   https://www.boost.org/libs/mpl/doc/refmanual/integral-constant.html
+// https://www.boost.org/libs/mpl/doc/refmanual/integral-constant.html
 template <bool b>
 struct Boolean_tag {
   typedef boost::mpl::integral_c_tag tag;
@@ -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>