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"