Define and use CGAL_NO_ATOMIC

Box_intersection_d/include/CGAL/Box_intersection_d/Box_d.h

@@ -39,7 +39,11 @@ template <class Dummy>
 struct Unique_numbers {
     typedef std::size_t ID;
     Unique_numbers() {
+#ifdef CGAL_NO_ATOMIC
+      static std::size_t n = 0;
+#else
       static CGAL::cpp11::atomic<std::size_t> n; // initialized to 0
+#endif
       i = n++;
     }
     std::size_t id() const { return i; }

Installation/include/CGAL/atomic.h

@@ -30,7 +30,7 @@
 #      include <boost/atomic.hpp>
 #      define CGAL_ATOMIC_NS boost
 #    else // BOOST_VERSION < 105300
-#      define CGAL_NO_ATOMIC "Boost.Atomic was introduced in Boost-1.53"
+#      define CGAL_NO_ATOMIC "Boost.Atomic was introduced in Boost-1.53".
 #    endif // BOOST_VERSION < 105300
 #  endif // not CGAL_CAN_USE_CXX11_ATOMIC
 
@@ -50,7 +50,8 @@
      }
    }
 #  endif // CGAL_ATOMIC_NS
-
+#else
+#  define CGAL_NO_ATOMIC "No atomic because CGAL_NO_THREADS is defined."
 #endif // CGAL_HAS_THREADS
 
 #endif // CGAL_ATOMIC_H