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>

Makefile

@@ -1687,11 +1687,21 @@ ifeq ($(uname_S),Darwin)
 			BASIC_CFLAGS += -I/sw/include
 			BASIC_LDFLAGS += -L/sw/lib
                 endif
+                ifeq ($(shell test -d /opt/sw/lib && echo y),y)
+			BASIC_CFLAGS += -I/opt/sw/include
+			BASIC_LDFLAGS += -L/opt/sw/lib
+			ifeq ($(shell test -e /opt/sw/lib/libiconv.dylib && echo y),y)
+				HAS_GOOD_LIBICONV = Yes
+			endif
+                endif
         endif
         ifndef NO_DARWIN_PORTS
                 ifeq ($(shell test -d /opt/local/lib && echo y),y)
 			BASIC_CFLAGS += -I/opt/local/include
 			BASIC_LDFLAGS += -L/opt/local/lib
+			ifeq ($(shell test -e /opt/local/lib/libiconv.dylib && echo y),y)
+				HAS_GOOD_LIBICONV = Yes
+			endif
                 endif
         endif
         ifndef NO_APPLE_COMMON_CRYPTO
@@ -1714,6 +1724,7 @@ endif
 ifdef USE_HOMEBREW_LIBICONV
 ifeq ($(shell test -d $(HOMEBREW_PREFIX)/opt/libiconv && echo y),y)
 	ICONVDIR ?= $(HOMEBREW_PREFIX)/opt/libiconv
+	HAS_GOOD_LIBICONV = Yes
 endif
 endif
 endif
@@ -1859,6 +1870,11 @@ ifndef NO_ICONV
                 endif
 		EXTLIBS += $(ICONV_LINK) -liconv
         endif
+        ifdef NEEDS_GOOD_LIBICONV
+        ifndef HAS_GOOD_LIBICONV
+                BASIC_CFLAGS += -DICONV_RESTART_RESET
+        endif
+        endif
 endif
 ifdef ICONV_OMITS_BOM
 	BASIC_CFLAGS += -DICONV_OMITS_BOM

config.mak.uname

@@ -157,6 +157,7 @@ ifeq ($(uname_S),Darwin)
         endif
         ifeq ($(shell test "$(DARWIN_MAJOR_VERSION)" -ge 24 && echo 1),1)
 		USE_HOMEBREW_LIBICONV = UnfortunatelyYes
+		NEEDS_GOOD_LIBICONV = UnfortunatelyYes
         endif
 
 	# The builtin FSMonitor on MacOS builds upon Simple-IPC.  Both require