Added Boost.Predef to check environment such as endian.

https://github.com/boostorg/predef.git has been added as a submodule on
external/boost/predef.

In order to avoid macro name conflicts, replaced "BOOST_" with "MSGPACK_"
and "boost/" with "msgpack/" then copy replaced files to include/msgpack.
This process is described in CMakeLists.txt.
Replaced files are included as a part of msgpack-c. So you don't need to
do the converting process each time.

Fixed endian checking logic.

include/msgpack/predef/library/c/zos.h

@@ -0,0 +1,57 @@
+/*
+Copyright Rene Rivera 2008-2013
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE_1_0.txt or copy at
+http://www.boost.org/LICENSE_1_0.txt)
+*/
+
+#ifndef MSGPACK_PREDEF_LIBRARY_C_ZOS_H
+#define MSGPACK_PREDEF_LIBRARY_C_ZOS_H
+
+#include <msgpack/predef/library/c/_prefix.h>
+
+#include <msgpack/predef/version_number.h>
+#include <msgpack/predef/make.h>
+
+/*`
+[heading `MSGPACK_LIB_C_ZOS`]
+
+z/OS libc Standard C library.
+Version number available as major, minor, and patch.
+
+[table
+    [[__predef_symbol__] [__predef_version__]]
+
+    [[`__LIBREL__`] [__predef_detection__]]
+
+    [[`__LIBREL__`] [V.R.P]]
+    [[`__TARGET_LIB__`] [V.R.P]]
+    ]
+ */
+
+#define MSGPACK_LIB_C_ZOS MSGPACK_VERSION_NUMBER_NOT_AVAILABLE
+
+#if defined(__LIBREL__)
+#   undef MSGPACK_LIB_C_ZOS
+#   if !defined(MSGPACK_LIB_C_ZOS) && defined(__LIBREL__)
+#       define MSGPACK_LIB_C_ZOS MSGPACK_PREDEF_MAKE_0X_VRRPPPP(__LIBREL__)
+#   endif
+#   if !defined(MSGPACK_LIB_C_ZOS) && defined(__TARGET_LIB__)
+#       define MSGPACK_LIB_C_ZOS MSGPACK_PREDEF_MAKE_0X_VRRPPPP(__TARGET_LIB__)
+#   endif
+#   if !defined(MSGPACK_LIB_C_ZOS)
+#       define MSGPACK_LIB_C_ZOS MSGPACK_VERSION_NUMBER_AVAILABLE
+#   endif
+#endif
+
+#if MSGPACK_LIB_C_ZOS
+#   define MSGPACK_LIB_C_ZOS_AVAILABLE
+#endif
+
+#define MSGPACK_LIB_C_ZOS_NAME "z/OS"
+
+#include <msgpack/predef/detail/test.h>
+MSGPACK_PREDEF_DECLARE_TEST(MSGPACK_LIB_C_ZOS,MSGPACK_LIB_C_ZOS_NAME)
+
+
+#endif