autoconf: Add support for setting NO_ICONV and ICONVDIR

autoconf: Add support for setting NO_ICONV and ICONVDIR

Add support for ./configure options --without-iconv (if neither libc
nor libiconv properly support iconv), and for --with-iconv=PATH (to
set prefix to libiconv library and headers, used only when
NEED_LIBICONV is set).  While at it, make ./configure set or unset
NO_ICONV always (it is not autodetected in Makefile).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>