[patch] Find if we are big endian at compile time

[Rafael Espíndola]

> sys/endian.h is the prefered public interface, but I forgot if there is
> any version we should care about that doesn't have it. I'd go as far as
> just using that one and see if anyone complains.

The problem is that sys/endian.h is not available on OS X. If we can
keep this down to 3 cases (linux, *bsd, windows), that is probably
better than 4 (linux, os x, *bsd, windows).

> Joerg

Cheers,
Rafael