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.

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

Makefile

@@ -1692,6 +1692,7 @@ ifeq ($(uname_S),Darwin)
                 ifeq ($(shell test -d /opt/local/lib && echo y),y)
 			BASIC_CFLAGS += -I/opt/local/include
 			BASIC_LDFLAGS += -L/opt/local/lib
+			HAS_GOOD_LIBICONV = Yes
                 endif
         endif
         ifndef NO_APPLE_COMMON_CRYPTO
@@ -1714,6 +1715,7 @@ 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 +1861,11 @@ 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