contrib/buildsystems: handle libiconv, too

contrib/buildsystems: handle libiconv, too

Git's test suite shows tons of breakages unless Git is compiled
*without* NO_ICONV. That means, in turn, that we need to generate
build definitions *with* libiconv, which in turn implies that we
have to handle the -liconv option properly.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>