Use unique_ptr instead of auto_ptr when possible

Previously we only used it on GCC 6 to avoid deprecated declaration
warnings. Now we are proactive and use it whenever compiling as C++11
(or MSVC2010+).

It also moves the logic for deciding between unique_ptr and auto_ptr
into a single location in config.h.

This fixes some use cases that were previously broken, including:
* CXX=clang++ -std=c++11 -Werror=deprecated-declarations
* CXX=g++-6 -std=c++03 -Werror=deprecated-declarations

Author: AbigailBuccaneer

Diff for: include/json/config.h

@@ -51,6 +51,16 @@
 #define JSON_API
 #endif
 
+#if !defined(JSON_HAS_UNIQUE_PTR)
+#if __cplusplus >= 201103L
+#define JSON_HAS_UNIQUE_PTR (1)
+#elif _MSC_VER >= 1600
+#define JSON_HAS_UNIQUE_PTR (1)
+#else
+#define JSON_HAS_UNIQUE_PTR (0)
+#endif
+#endif
+
 // If JSON_NO_INT64 is defined, then Json only support C++ "int" type for
 // integer
 // Storages, and 64 bits integer support is disabled.

Diff for: src/lib_json/json_reader.cpp

@@ -43,7 +43,7 @@ static int       stackDepth_g = 0;  // see readValue()
 
 namespace Json {
 
-#if __GNUC__ >= 6
+#if JSON_HAS_UNIQUE_PTR
 typedef std::unique_ptr<CharReader> const  CharReaderPtr;
 #else
 typedef std::auto_ptr<CharReader>          CharReaderPtr;

Diff for: src/lib_json/json_writer.cpp

@@ -54,7 +54,7 @@
 
 namespace Json {
 
-#if __GNUC__ >= 6
+#if JSON_HAS_UNIQUE_PTR
 typedef std::unique_ptr<StreamWriter> const  StreamWriterPtr;
 #else
 typedef std::auto_ptr<StreamWriter>          StreamWriterPtr;