include/json: check MSVC before adding pragma warning directive

* File changes: include/json/{config.h, writer.h}
* Summary: previously JSONCPP_DISABLE_DLL_INTERFACE_WARNING is defined
  when both _MSC_VER or __MINGW32__ defined. So, when compiled with
  GCC or Clang using MinGW toolchain those #pragma warnings are also
  enabled.
  This change enables those #prgma warnings when only MSVC is used
  i.e. only _MSC_VER is defined.

Author: Biswa96

include/json/config.h

@@ -34,18 +34,21 @@
 #if defined(JSON_DLL_BUILD)
 #if defined(_MSC_VER) || defined(__MINGW32__)
 #define JSON_API __declspec(dllexport)
-#define JSONCPP_DISABLE_DLL_INTERFACE_WARNING
 #elif defined(__GNUC__) || defined(__clang__)
 #define JSON_API __attribute__((visibility("default")))
 #endif // if defined(_MSC_VER)
 
 #elif defined(JSON_DLL)
 #if defined(_MSC_VER) || defined(__MINGW32__)
 #define JSON_API __declspec(dllimport)
-#define JSONCPP_DISABLE_DLL_INTERFACE_WARNING
 #endif // if defined(_MSC_VER)
 #endif // ifdef JSON_DLL_BUILD
 
+// Disable MSVC specific compiler warnings
+#if defined(_MSC_VER)
+#define JSONCPP_DISABLE_DLL_INTERFACE_WARNING
+#endif
+
 #if !defined(JSON_API)
 #define JSON_API
 #endif

include/json/writer.h

@@ -15,7 +15,7 @@
 
 // Disable warning C4251: <data member>: <type> needs to have dll-interface to
 // be used by...
-#if defined(JSONCPP_DISABLE_DLL_INTERFACE_WARNING) && defined(_MSC_VER)
+#if defined(JSONCPP_DISABLE_DLL_INTERFACE_WARNING)
 #pragma warning(push)
 #pragma warning(disable : 4251)
 #endif // if defined(JSONCPP_DISABLE_DLL_INTERFACE_WARNING)