Fix preprocessor warnings about undefined _MSVC_LANG

Stricter compiler/settings, such as found during a build
on FreeBSD with clang 14, issue warnings of the kind below.

/usr/local/include/exiv2/value.hpp:1272:31: warning: '_MSVC_LANG' is not defined, evaluates to 0 [-Wundef]

Fix: Guard use of _MSVC_LANG by a check.

Personally, I found that MSVC has several feature-specific
checks in predefined macros which might allow for one
standards-based check that matches GCC/clang/MSVC rather than the
split check for C++ standard and MSVC language version settings.

See https://en.cppreference.com/w/cpp/feature_test

I am not building Exiv2 on MSVC, so I cannot test/suggest
anything here.

include/exiv2/slice.hpp

@@ -255,7 +255,7 @@ struct ContainerStorage {
   using iterator = typename container::iterator;
   using const_iterator = typename container::const_iterator;
 
-#if __cplusplus >= 201402L || _MSVC_LANG >= 201402L
+#if __cplusplus >= 201402L || (defined(_MSVC_LANG) && (_MSVC_LANG >= 201402L))
   using value_type = std::remove_cv_t<typename container::value_type>;
 #else
   using value_type = typename std::remove_cv<typename container::value_type>::type;
@@ -320,7 +320,7 @@ struct ContainerStorage {
  */
 template <typename storage_type>
 struct PtrSliceStorage {
-#if __cplusplus >= 201402L || _MSVC_LANG >= 201402L
+#if __cplusplus >= 201402L || (defined(_MSVC_LANG) && (_MSVC_LANG >= 201402L))
   using value_type = std::remove_cv_t<std::remove_pointer_t<storage_type>>;
 #else
   using value_type = typename std::remove_cv<typename std::remove_pointer<storage_type>::type>::type;
@@ -423,7 +423,7 @@ struct Slice : public Internal::MutableSliceBase<Internal::ContainerStorage, con
   using iterator = typename container::iterator;
   using const_iterator = typename container::const_iterator;
 
-#if __cplusplus >= 201402L || _MSVC_LANG >= 201402L
+#if __cplusplus >= 201402L || (defined(_MSVC_LANG) && (_MSVC_LANG >= 201402L))
   using value_type = std::remove_cv_t<typename container::value_type>;
 #else
   using value_type = typename std::remove_cv<typename container::value_type>::type;
@@ -460,7 +460,7 @@ struct Slice<const container> : public Internal::ConstSliceBase<Internal::Contai
   using iterator = typename container::iterator;
   using const_iterator = typename container::const_iterator;
 
-#if __cplusplus >= 201402L || _MSVC_LANG >= 201402L
+#if __cplusplus >= 201402L || (defined(_MSVC_LANG) && (_MSVC_LANG >= 201402L))
   using value_type = std::remove_cv_t<typename container::value_type>;
 #else
   using value_type = typename std::remove_cv<typename container::value_type>::type;

include/exiv2/value.hpp

@@ -1254,7 +1254,7 @@ class ValueType : public Value {
     } else if (std::is_signed<I>::value) {
 #endif
       // conversion is from unsigned to signed
-#if __cplusplus >= 201402L || _MSVC_LANG >= 201402L
+#if __cplusplus >= 201402L || (defined(_MSVC_LANG) && (_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());
@@ -1269,7 +1269,7 @@ class ValueType : public Value {
         return 0;
       }
       // Inputs are not negative so convert them to unsigned.
-#if __cplusplus >= 201402L || _MSVC_LANG >= 201402L
+#if __cplusplus >= 201402L || (defined(_MSVC_LANG) && (_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