Interix identification.

[Donn Terry]

I note (with pleasure) that config.guess now knows about Interix. One
thing that it would have been nice for it to do at the same time is
automatically add a #define _ALL_SOURCE everywhere.  The code to do
exactly the correct thing is there as a special case for AIX (see
lib/autoconf/specific.m4).  Recoginzing __INTERIX (two underscores) in
the same way that _AIX is already recognized (as in the below taken from
specific.m4) would be very helpful.  (_ALL_SOURCE has "the same" meaning
on AIX and Interix: allow everything, rather than just POSIX.)

AC_EGREP_CPP(yes,
[#ifdef _AIX
  yes
#endif
],

(Just to avoid IP issues, this is small enough that I'd rather describe
it than provide an actual patch.)

(Whether it's best to do this as a variant on the AIX macro, or bury it
in the existing AIX macro, or add a new "needs _ALL_SOURCE" macro is a
judgement call I leave to you.)

Donn