Fix a warning when `FT` is `int`

https://cgal.geometryfactory.com/CGAL/testsuite/CGAL-4.14-Ic-63/Arrangement_on_surface_2_Examples/TestReport_Blake_Windows_MSVCPreview-Release-64bits.gz

Cartesian_kernel/include/CGAL/constructions/kernel_ftC2.h

@@ -28,6 +28,7 @@
 
 #include <CGAL/determinant.h>
 #include <CGAL/number_utils.h>
+#include <boost/type_traits/is_integral.hpp>
 
 namespace CGAL {
 
@@ -284,6 +285,15 @@ line_get_pointC2(const FT &a, const FT &b, const FT &c, int i,
 {
   if (CGAL_NTS is_zero(b))
     {
+      // Laurent Rineau, 2018/12/07: I had this CGAL_assume to calm
+      // down a warning from MSVC 2017:
+      // > include\cgal\constructions\kernel_ftc2.h(287) :
+      // >   warning C4723: potential divide by 0
+      // The test `!boost::is_integral<FT>::value` is there to avoid
+      // that `a != 0` is tested on anything but integral types, for
+      // performance reasons.
+      CGAL_assume(!boost::is_integral<FT>::value || a != FT(0));
+
       x = -c/a;
       y = 1 - i * a;
     }