Make it a bit more multithreading friendly

Author: jia3ep

src/lib_json/json_reader.cpp

@@ -45,7 +45,11 @@
 #endif
 
 static int const stackLimit_g = 1000;
+#if __cplusplus >= 201103L
+thread_local static int stackDepth_g = 0;  // see readValue()
+#else
 static int       stackDepth_g = 0;  // see readValue()
+#endif
 
 namespace Json {