system/ByteOrder: fix byte order detection on FreeBSD/aarch64

Patch from
 https://svnweb.freebsd.org/ports/head/audio/musicpd/files/patch-src_system_ByteOrder.hxx?revision=441921&view=co
(with a tiny modification)

Closes #59

src/system/ByteOrder.hxx

@@ -52,6 +52,15 @@
 #    define IS_LITTLE_ENDIAN false
 #    define IS_BIG_ENDIAN true
 #  endif
+#elif defined(__BYTE_ORDER__)
+/* GCC-specific macros */
+#  if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#    define IS_LITTLE_ENDIAN true
+#    define IS_BIG_ENDIAN false
+#  else
+#    define IS_LITTLE_ENDIAN false
+#    define IS_BIG_ENDIAN true
+#  endif
 #else
 /* generic compile-time check */
 #  include <endian.h>