document.h: define __STDC_CONSTANT_MACROS

The C++ standard does not include the C99 macros used to set the (U)INT64
constants in document.h and reader.h (see adf66292 and ce1fece2).

Many implementations include their definition when the
__STDC_CONSTANT_MACROS preprocessor symbol is defined.

See e.g. http://www.cprogramdevelop.com/5272623/,
needed to successfully build in travis-ci.org's environment.

include/rapidjson/rapidjson.h

@@ -10,13 +10,19 @@
 ///////////////////////////////////////////////////////////////////////////////
 // RAPIDJSON_NO_INT64DEFINE
 
-// Here defines int64_t and uint64_t types in global namespace.
+// Here defines int64_t and uint64_t types in global namespace as well as the
+// (U)INT64_C constant macros.
 // If user have their own definition, can define RAPIDJSON_NO_INT64DEFINE to disable this.
 #ifndef RAPIDJSON_NO_INT64DEFINE
+#ifndef __STDC_CONSTANT_MACROS
+#  define __STDC_CONSTANT_MACROS 1 // required by C++ standard
+#endif
 #ifdef _MSC_VER
+#include "msinttypes/stdint.h"
 #include "msinttypes/inttypes.h"
 #else
 // Other compilers should have this.
+#include <stdint.h>
 #include <inttypes.h>
 #endif
 #endif // RAPIDJSON_NO_INT64TYPEDEF