More endian detection

Since __BYTE_ORDER__ only avaliable since GCC 4.6.0, add more detection.

include/rapidjson/rapidjson.h

@@ -52,15 +52,16 @@
 #define RAPIDJSON_BIGENDIAN		1	//!< Big endian machine
 
 //! Endianness of the machine.
-/*!	GCC provided macro for detecting endianness of the target machine. But other
+/*!	GCC 4.6 provided macro for detecting endianness of the target machine. But other
 	compilers may not have this. User can define RAPIDJSON_ENDIAN to either
 	\ref RAPIDJSON_LITTLEENDIAN or \ref RAPIDJSON_BIGENDIAN.
 
 	Implemented with reference to 
-	https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html
+	https://gcc.gnu.org/onlinedocs/gcc-4.6.0/cpp/Common-Predefined-Macros.html
 	http://www.boost.org/doc/libs/1_42_0/boost/detail/endian.hpp
 */
 #ifndef RAPIDJSON_ENDIAN
+// Detect with GCC 4.6's macro
 #  ifdef __BYTE_ORDER__
 #    if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
 #      define RAPIDJSON_ENDIAN RAPIDJSON_LITTLEENDIAN
@@ -69,10 +70,22 @@
 #    else
 #      error Unknown machine endianess detected. User needs to define RAPIDJSON_ENDIAN.
 #	 endif // __BYTE_ORDER__
+// Detect with GLIBC's endian.h
+#  elif defined(__GLIBC__)
+#    include <endian.h>
+#    if (__BYTE_ORDER == __LITTLE_ENDIAN)
+#      define RAPIDJSON_ENDIAN RAPIDJSON_LITTLEENDIAN
+#	 elif (__BYTE_ORDER == __BIG_ENDIAN)
+#      define RAPIDJSON_ENDIAN RAPIDJSON_BIGENDIAN
+#	 else
+#      error Unknown machine endianess detected. User needs to define RAPIDJSON_ENDIAN.
+#   endif // __GLIBC__
+// Detect with _LITTLE_ENDIAN and _BIG_ENDIAN macro
 #  elif defined(_LITTLE_ENDIAN) && !defined(_BIG_ENDIAN)
 #	 define RAPIDJSON_ENDIAN RAPIDJSON_LITTLEENDIAN
 #  elif defined(_BIG_ENDIAN) && !defined(_LITTLE_ENDIAN)
 #	 define RAPIDJSON_ENDIAN RAPIDJSON_BIGENDIAN
+// Detect with architecture macros
 #  elif defined(__sparc) || defined(__sparc__) || defined(_POWER) || defined(__powerpc__) || defined(__ppc__) || defined(__hpux) || defined(__hppa) || defined(_MIPSEB) || defined(_POWER) || defined(__s390__)
 #	 define RAPIDJSON_ENDIAN RAPIDJSON_BIGENDIAN
 #  elif defined(__i386__) || defined(__alpha__) || defined(__ia64) || defined(__ia64__) || defined(_M_IX86) || defined(_M_IA64) || defined(_M_ALPHA) || defined(__amd64) || defined(__amd64__) || defined(_M_AMD64) || defined(__x86_64) || defined(__x86_64__) || defined(_M_X64) || defined(__bfin__)