diff --git a/Installation/include/CGAL/config.h b/Installation/include/CGAL/config.h
index 432229f8e2d..8206195e6b2 100644
--- a/Installation/include/CGAL/config.h
+++ b/Installation/include/CGAL/config.h
@@ -354,6 +354,13 @@ using std::max;
 #  define CGAL_UNUSED
 #endif
 
+// Macro to specify that gcc's pedantic mode should not warn.
+#ifdef __GNUG__
+#  define CGAL_GNU_EXTENSION  __extension__
+#else
+#  define CGAL_GNU_EXTENSION
+#endif
+
 // Macro CGAL_ASSUME
 // Call a builtin of the compiler to pass a hint to the compiler
 #if __has_builtin(__builtin_unreachable) || (CGAL_GCC_VERSION >= 40500 && !__STRICT_ANSI__)
diff --git a/Number_types/include/CGAL/long_long.h b/Number_types/include/CGAL/long_long.h
index 5562fee1d36..ea98538a058 100644
--- a/Number_types/include/CGAL/long_long.h
+++ b/Number_types/include/CGAL/long_long.h
@@ -88,9 +88,9 @@ template <> class Real_embeddable_traits< unsigned long long >
     };
 };
 
-#ifdef __SIZEOF_INT128__
+#ifdef BOOST_HAS_INT128
 // __int128
-__extension__ // For gcc's pedantic mode
+CGAL_GNU_EXTENSION
 template<> class Algebraic_structure_traits< __int128 >
   : public Algebraic_structure_traits_base< __int128,
                                             Euclidean_ring_tag > {
@@ -104,7 +104,7 @@ template<> class Algebraic_structure_traits< __int128 >
 
 };
 
-__extension__ // For gcc's pedantic mode
+CGAL_GNU_EXTENSION
 template <> class Real_embeddable_traits< __int128 >
   : public INTERN_RET::Real_embeddable_traits_base< __int128 , CGAL::Tag_true > {
   public:
@@ -119,7 +119,7 @@ template <> class Real_embeddable_traits< __int128 >
 };
 
 // unsigned __int128
-__extension__ // For gcc's pedantic mode
+CGAL_GNU_EXTENSION
 template <> class Real_embeddable_traits< unsigned __int128 >
   : public INTERN_RET::Real_embeddable_traits_base< unsigned __int128 , CGAL::Tag_true > {
   public: