COMP: Prefer the C++ headers over the C99 headers

Using the C++11 headers keeps the library cleaner and more
rigorously scoped use of namespaces.
diff --git a/include/json/assertions.h b/include/json/assertions.h
index 482c4ca..9bf076f 100644
--- a/include/json/assertions.h
+++ b/include/json/assertions.h
@@ -7,7 +7,7 @@
 #define CPPTL_JSON_ASSERTIONS_H_INCLUDED
 
 #include <sstream>
-#include <stdlib.h>
+#include <cstdlib>
 
 #if !defined(JSON_IS_AMALGAMATION)
 #include "config.h"
diff --git a/include/json/config.h b/include/json/config.h
index b0ecfa2..a36ca15 100644
--- a/include/json/config.h
+++ b/include/json/config.h
@@ -5,8 +5,8 @@
 
 #ifndef JSON_CONFIG_H_INCLUDED
 #define JSON_CONFIG_H_INCLUDED
-#include <stddef.h>
-#include <stdint.h> //typedef int64_t, uint64_t
+#include <cstddef>
+#include <cstdint> //typedef int64_t, uint64_t
 #include <string>   //typedef String
 
 /// If defined, indicates that json library is embedded in CppTL library.