Makefile: make NO_ICONV really mean "no iconv"

The Makefile tweak NO_ICONV is meant to allow Git to be built without iconv in case iconv is not installed or is otherwise dysfunctional. However, NO_ICONV's disabling of iconv is incomplete and can incorrectly allow "-liconv" to slip into the linker flags when NEEDS_LIBICONV is defined, which breaks the build when iconv is not installed. On some platforms, iconv lives directly in libc, whereas, on others it resides in libiconv. For the latter case, NEEDS_LIBICONV instructs the Makefile to add "-liconv" to the linker flags. config.mak.uname automatically defines NEEDS_LIBICONV for platforms which require it. The adding of "-liconv" is done unconditionally, despite NO_ICONV. Work around this problem by making NO_ICONV take precedence over NEEDS_LIBICONV. Reported by: Mahmoud Al-Qudsi <mqudsi@neosmart.net> Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>