endian: New module.

[Collin Funk]

I haven't pushed this since it is my first time attempting the gnulib
header wizardry and doing anything of value in Autoconf.

The configure check should check for unique values of LITTLE_ENDIAN
and BIG_ENDIAN, and that BYTE_ORDER is defined to one of them. POSIX
doesn't require PDP_ENDIAN and I would rather not think about it. :)

Then it checks for uint16_t, uint32_t, and uint64_t being defined and
the existence of the be16toh macros/functions and friends.

Let me know if I missed anything. Other than the docs and test module
which I can do later today (assuming this patch is decent enough).

Collin

0001-endian-New-module.patch
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