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/compiler/clang.h

@@ -0,0 +1,57 @@
+/*
+Copyright Rene Rivera 2008-2014
+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_COMPILER_CLANG_H
+#define MSGPACK_PREDEF_COMPILER_CLANG_H
+
+#include <msgpack/predef/version_number.h>
+#include <msgpack/predef/make.h>
+
+/*`
+[heading `MSGPACK_COMP_CLANG`]
+
+[@http://en.wikipedia.org/wiki/Clang Clang] compiler.
+Version number available as major, minor, and patch.
+
+[table
+    [[__predef_symbol__] [__predef_version__]]
+
+    [[`__clang__`] [__predef_detection__]]
+
+    [[`__clang_major__`, `__clang_minor__`, `__clang_patchlevel__`] [V.R.P]]
+    ]
+ */
+
+#define MSGPACK_COMP_CLANG MSGPACK_VERSION_NUMBER_NOT_AVAILABLE
+
+#if defined(__clang__)
+#   define MSGPACK_COMP_CLANG_DETECTION MSGPACK_VERSION_NUMBER(__clang_major__,__clang_minor__,__clang_patchlevel__)
+#endif
+
+#ifdef MSGPACK_COMP_CLANG_DETECTION
+#   if defined(MSGPACK_PREDEF_DETAIL_COMP_DETECTED)
+#       define MSGPACK_COMP_CLANG_EMULATED MSGPACK_COMP_CLANG_DETECTION
+#   else
+#       undef MSGPACK_COMP_CLANG
+#       define MSGPACK_COMP_CLANG MSGPACK_COMP_CLANG_DETECTION
+#   endif
+#   define MSGPACK_COMP_CLANG_AVAILABLE
+#   include <msgpack/predef/detail/comp_detected.h>
+#endif
+
+#define MSGPACK_COMP_CLANG_NAME "Clang"
+
+#include <msgpack/predef/detail/test.h>
+MSGPACK_PREDEF_DECLARE_TEST(MSGPACK_COMP_CLANG,MSGPACK_COMP_CLANG_NAME)
+
+#ifdef MSGPACK_COMP_CLANG_EMULATED
+#include <msgpack/predef/detail/test.h>
+MSGPACK_PREDEF_DECLARE_TEST(MSGPACK_COMP_CLANG_EMULATED,MSGPACK_COMP_CLANG_NAME)
+#endif
+
+
+#endif