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