use some future C++ stuff when possible

Signed-off-by: Rosen Penev <rosenp@gmail.com>

include/exiv2/value.hpp

@@ -1270,16 +1270,28 @@ class ValueType : public Value {
     }
 
     // Check for integer overflow.
+#if __cplusplus >= 201703L
+    if (std::is_signed_v<I> == std::is_signed_v<decltype(a)>) {
+#else
     if (std::is_signed<I>::value == std::is_signed<decltype(a)>::value) {
+#endif
       // conversion does not change sign
       const auto imin = std::numeric_limits<I>::min();
       const auto imax = std::numeric_limits<I>::max();
       if (imax < b || a < imin || imax < a) {
         return 0;
       }
+#if __cplusplus >= 201703L
+    } else if (std::is_signed_v<I>) {
+#else
     } else if (std::is_signed<I>::value) {
+#endif
       // conversion is from unsigned to signed
+#if __cplusplus >= 201402L
+      const auto imax = std::make_unsigned_t<I>(std::numeric_limits<I>::max());
+#else
       const auto imax = typename std::make_unsigned<I>::type(std::numeric_limits<I>::max());
+#endif
       if (imax < b || imax < a) {
         return 0;
       }
@@ -1290,8 +1302,13 @@ class ValueType : public Value {
         return 0;
       }
       // Inputs are not negative so convert them to unsigned.
+#if __cplusplus >= 201402L
+      const auto a_u = std::make_unsigned_t<decltype(a)>(a);
+      const auto b_u = std::make_unsigned_t<decltype(b)>(b);
+#else
       const auto a_u = typename std::make_unsigned<decltype(a)>::type(a);
       const auto b_u = typename std::make_unsigned<decltype(b)>::type(b);
+#endif
       if (imax < b_u || imax < a_u) {
         return 0;
       }