Expand pointer test to fix build on Clang as well.

Building in -Wall -Werror in c++11 mode errors on those auto_ptrs,
at least on Clang 3.7.x on Debian Jessie.

Author: rpavlik

src/lib_json/json_reader.cpp

@@ -45,6 +45,8 @@ namespace Json {
 
 #if defined(JSON_USE_SCOPED_PTR)
 typedef std::scoped_ptr<CharReader> const  CharReaderPtr;
+#elif defined(JSON_USE_UNIQUE_PTR)
+typedef std::unique_ptr<CharReader> const  CharReaderPtr;
 #else
 typedef std::auto_ptr<CharReader>          CharReaderPtr;
 #endif

src/lib_json/json_tool.h

@@ -12,8 +12,15 @@
  * It is an internal header that must not be exposed.
  */
 
+// Note that a C++ standard header must be included before this one (the code
+// below indicates that <string> would logically be included) in order to set
+// macros required to test for libstdc++
+
 #if defined(__GNUC__) && (__GNUC__ >= 6)
 #define JSON_USE_SCOPED_PTR
+#elif defined(__clang__) && defined(__USE_ISOCXX11) && defined(__GLIBCXX__)
+// clang with libstdc++
+#define JSON_USE_UNIQUE_PTR
 #endif
 
 namespace Json {

src/lib_json/json_writer.cpp

@@ -56,6 +56,8 @@ namespace Json {
 
 #if defined(JSON_USE_SCOPED_PTR)
 typedef std::scoped_ptr<StreamWriter> const  StreamWriterPtr;
+#elif defined(JSON_USE_UNIQUE_PTR)
+typedef std::unique_ptr<StreamWriter>        StreamWriterPtr;
 #else
 typedef std::auto_ptr<StreamWriter>          StreamWriterPtr;
 #endif