Add macro CGAL_MSVC_USE_STD_FABS

On Windows, with MSVC, std::fabs is an intrinsic function with /Oi (implied
with /O1, /O2, and so on) *if* /fp:strict is not used. We should reevaluate
the need to use /fp:strict (and maybe use /fp:precise, the default,
combined with #pragma fenv_access).

First step: bench! This macro will help.

Number_types/include/CGAL/double.h

@@ -124,7 +124,7 @@ template <> class Real_embeddable_traits< double >
   public:
 
 // GCC is faster with std::fabs().
-#ifdef __GNUG__
+#if defined(__GNUG__) || defined(CGAL_MSVC_USE_STD_FABS)
     class Abs
       : public std::unary_function< Type, Type > {
       public: