fix __cplusplus check for MSVC

Because of an MSVC bug, __cplusplus is always 199711L. Use _MSVC_LANG
which gets set properly.

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

include/exiv2/value.hpp

@@ -1270,7 +1270,7 @@ class ValueType : public Value {
     }
 
     // Check for integer overflow.
-#if __cplusplus >= 201703L
+#if __cplusplus >= 201703L || _MSVC_LANG >= 201703L
     if constexpr (std::is_signed_v<I> == std::is_signed_v<decltype(a)>) {
 #else
     if (std::is_signed<I>::value == std::is_signed<decltype(a)>::value) {
@@ -1281,13 +1281,13 @@ class ValueType : public Value {
       if (imax < b || a < imin || imax < a) {
         return 0;
       }
-#if __cplusplus >= 201703L
+#if __cplusplus >= 201703L || _MSVC_LANG >= 201703L
     } else if constexpr (std::is_signed_v<I>) {
 #else
     } else if (std::is_signed<I>::value) {
 #endif
       // conversion is from unsigned to signed
-#if __cplusplus >= 201402L
+#if __cplusplus >= 201402L || _MSVC_LANG >= 201402L
       const auto imax = static_cast<std::make_unsigned_t<I>>(std::numeric_limits<I>::max());
 #else
       const auto imax = static_cast<typename std::make_unsigned<I>::type>(std::numeric_limits<I>::max());
@@ -1302,7 +1302,7 @@ class ValueType : public Value {
         return 0;
       }
       // Inputs are not negative so convert them to unsigned.
-#if __cplusplus >= 201402L
+#if __cplusplus >= 201402L || _MSVC_LANG >= 201402L
       const auto a_u = static_cast<std::make_unsigned_t<decltype(a)>>(a);
       const auto b_u = static_cast<std::make_unsigned_t<decltype(b)>>(b);
 #else