Makefile: do not set NEEDS_LIBICONV for Solaris 8

Makefile: do not set NEEDS_LIBICONV for Solaris 8

This breaks my build on Solaris 8, as there is no separate
libiconv.

The history of this line is somewhat convoluted. In 2fd955c
(in November 2005), NEEDS_LIBICONV was turned on for all
Solaris builds, claiming to "fix an error in Solaris 10 by
setting NEEDS_LIBICONV".

Later, e15f545 (in February of 2006) claimed that "Solaris
9+ don't need iconv", and moved NEEDS_LIBICONV into a
section for Solaris 8.

Furthermore, Brandon Casey claims in

<5A1KxlhmUIHe8iXPxnXYuNXsq0Yjlbwkz2eBin3z7ELuL9nK-4tSpw@cipher.nrlssc.navy.mil>

that he does not set NEEDS_LIBICONV for Solaris 7.

So either one of those commits is totally wrong, or there is
some other magic going on where some Solaris installs need
it and others don't.

Given Brandon's statement and my problems on Solaris 8 with
NEEDS_LIBICONV, I am inclined to think the first commit was
bogus, and that NEEDS_LIBICONV shouldn't be set for Solaris
at all by default. If somebody wants to use iconv and has
installed it manually, they can set it in their config.mak.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>