fix detection of linux based os. According to https://sourceforge.net/p/predef/wiki/OperatingSystems/ 'linux' macro is obsolete, '__linux__' should be used instead. It turns out that 'linux' macro undefined when compiling project with e.g. '-std=c++11' compilation flag

Author: drdzyk

include/msgpack/predef/os/linux.h

@@ -27,7 +27,7 @@ Distributed under the Boost Software License, Version 1.0.
 #define MSGPACK_OS_LINUX MSGPACK_VERSION_NUMBER_NOT_AVAILABLE
 
 #if !defined(MSGPACK_PREDEF_DETAIL_OS_DETECTED) && ( \
-    defined(linux) || defined(__linux) \
+    defined(linux) || defined(__linux) || defined(__linux__) \
     )
 #   undef MSGPACK_OS_LINUX
 #   define MSGPACK_OS_LINUX MSGPACK_VERSION_NUMBER_AVAILABLE

include/msgpack/sysdep.hpp

@@ -88,7 +88,7 @@
 #elif defined(unix) || defined(__unix) || defined(__APPLE__) || defined(__OpenBSD__)
 
 #include <arpa/inet.h>  /* __BYTE_ORDER */
-#   if defined(linux)
+#   if defined(linux) || defined(__linux__)
 #       include <byteswap.h>
 #   endif