utf8.c: enable workaround for iconv under macOS 14/15

utf8.c: enable workaround for iconv under macOS 14/15

The previous commit introduced a workaround in utf8.c to deal
with broken iconv implementations.

It is enabled when a MacOS version is used that has a buggy
iconv library and there is no external library provided
(and linked against) from neither MacPorts nor Homebrew nor Fink.
For Homebrew, MacPorts and Fink we check if libiconv exist.
Introduce 2 new macros: HAS_GOOD_LIBICONV and NEEDS_GOOD_LIBICONV.

For Homebrew HAS_GOOD_LIBICONV is set when the libiconv directory
exist.
MacPorts can be installed with or without libiconv, so check if
libiconv.dylib exists (which is a softlink)

Fink compiles and installs libiconv by default.
Note that a fresh installation of Fink now defaults to /opt/sw.
Older versions used /sw as default, so leave the check and setting
of BASIC_CFLAGS and BASIC_LDFLAGS as is.
For the new default check for the existance of /opt/sw as well.
Add a check for /opt/sw/lib/libiconv.dylib which sets HAS_GOOD_LIBICONV

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>