Allow building Git in systems without iconv

Systems using some uClibc versions do not properly support
iconv stuff. This patch allows Git to be built on those
systems by passing NO_ICONV=YesPlease to make. The only
drawback is mailinfo won't do charset conversion in those
systems.

Signed-off-by: Fernando J. Pereda <ferdy@gentoo.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>