Fall back to naive built-in byte-swap routines

EL5 lacks htobe64 and friends.  Using its byteswap.h requires an endianness
check in our headers, which is fragile and compiler-dependent.  It's a fair
amount of code [1], and is only useful on EL5.

So instead, let's include a fallback implementation of these functions, and use
it whenever we can't find the (now-)standard Linux or OS X routines.  The
fallback is endianness-independent and should work on any platform.

[1] https://github.com/rurban/mosh/commit/da1a5abd1e09f0c1e9295357bb480a643d9dc8e3

configure.ac

@@ -272,7 +272,7 @@ AC_CHECK_DECL([htobe64],
   [AC_CHECK_DECL([OSSwapHostToBigInt64],
      [AC_DEFINE([HAVE_OSX_SWAP], [1],
         [Define if OSSwapHostToBigInt64 and friends exist.])],
-     [AC_MSG_ERROR([Unable to find byte swapping functions])],
+     [AC_MSG_WARN([Unable to find byte swapping functions; using built-in routines.])],
      [[#include <libkern/OSByteOrder.h>]])],
   [[#if defined(HAVE_ENDIAN_H)
 #include <endian.h>