NEWS: IRIX is still supported; only SGI C compiler depcomp no longer is

NEWS: IRIX is still supported; only SGI C compiler depcomp no longer is

This change is for the maint branch.

The only IRIX-specific support that is going to actually be removed in
the next major Automake versions is the depcomp support for the SGI
compiler.  That means that automatic dependency tracking will no
longer work with that compiler, but "normal" compilation should still
work, at least until the compiler is supported by Autoconf.

So there is no point in alarming our users by stating in the NEWS file
that "support for IRIX and the SGI compilers is going to be removed";
after all, while we don't test on nor particularly care about IRIX
anymore, that doesn't mean we are deliberately breaking it, and the
likelihood of an intended breakage there is very low.

See:
<http://lists.gnu.org/archive/html/automake-patches/2013-01/msg00164.html>
<http://lists.gnu.org/archive/html/automake-patches/2013-01/msg00165.html>

* NEWS (Future backward-incompatibilities: Adjust.  Improve explanation
of our rationale for dropping SGI support. Fix an unrelated typo while
at it.

Suggested-by: Peter Rosin <peda@lysator.liu.se>
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>