iconv issues, 4.2 branch

[Oliver Kiddle <okiddle@xxxxxxxxxxx>]

Having been away, I've not really followed the autoconf issues with
detecting iconv on Cygwin and Mac OS X but I thought I should mention
that GNU gettext comes with a file normally installed as
/usr/share/aclocal/iconv.m4. If someone wants to look at it, it might do
a better job of handling the different situations. It also detects the
varying prototypes which currently causes compiler warnings on some
systems.

I didn't use it at the time because it seemed overkill. It also appeared
to assume that iconv was essential, printing messages like "consider
installing GNU libiconv". This isn't really appropriate for zsh given
that, in many cases, \u/\U will work without needing iconv
(specifically  __STDC_ISO_10646__ plus wctomb and wchar_t available). So
it may just be that some ideas can be taken from iconv.m4.

Also, I've got a question about the zsh-4_2-patches branch? What is the
plan: is all the unicode stuff going to form a 4.3.0 release once ready
for testing with further 4.2.x releases coming on the 4_2 branch. It'd
be useful to know just what sort of changes we should be applying to
both branches and which only to HEAD.

Oliver