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