w32: Always require libiconv.

* configure.ac (missing_iconv): Set and die if we have no libiconv.
* m4/iconv.m4: Update from libiconv 1.14.
* tools/Makefile.am (gpgtar_LDADD): Add LIBICONV.
* jnlib/utf8conv.c: Always include iconv.h
(load_libiconv): Remove this w32 only function.
(iconv_open, iconv, iconv_close): Remove W32 function pointer.
(set_native_charset): Do not call load_libiconv.
(jnlib_iconv_open, jnlib_iconv, jnlib_iconv_close): Ditto.

This patch removes the on-demand-loading of libiconv which we did for
13 years or so. The rationale back then was that libiconv is too
large and often not used. Nowadays all kind of Unix software has been
ported to Windows and many of them require libiconv. Thus in the end
there is no saving from not requiring it. It also remove a common
source of trouble with awrong or missing iconv.dll.

• Signed-off-by: Werner Koch <wk@gnupg.org>