Use the c++17 constexpr if in CheckRange if possible (#753)

* change c++17 to use constexpr if in CheckRange

* remove unnecessary macro

* use the portable feature test macro rather than direct version check

include/gsl/span

@@ -556,7 +556,11 @@ private:
         // wrap around to a value always greater than size when casted.
 
         // check if we have enough space to wrap around
+#if defined(__cpp_if_constexpr)
+        if constexpr (sizeof(index_type) <= sizeof(size_t))
+#else
         if (sizeof(index_type) <= sizeof(size_t))
+#endif
         {
             return narrow_cast<size_t>(idx) < narrow_cast<size_t>(size);
         }