macOS: use iconv from Homebrew if needed and present

The library function iconv(3) supplied with macOS versions 15.7.2
(Sequoia) and 26.1 (Tahoe) is unreliable when doing conversions from
ISO-2022-JP to UTF-8 in multiple steps; t3900 reports this breakage:

  not ok 17 - ISO-2022-JP should be shown in UTF-8 now
  not ok 25 - ISO-2022-JP should be shown in UTF-8 now
  not ok 38 - commit --fixup into ISO-2022-JP from UTF-8

As a workaround, use libiconv from Homebrew, if available.  Search it in
its default locations: /opt/homebrew for Apple Silicon and /usr/local
for macOS Intel, with the former taking precedence.  Respect ICONVDIR if
already set by the user, though.

Helped-by: Koji Nakamaru <koji.nakamaru@gree.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rscharfe

Makefile

@@ -100,12 +100,15 @@ include shared.mak
 # specify your own (or DarwinPort's) include directories and
 # library directories by defining CFLAGS and LDFLAGS appropriately.
 #
-# Define NO_HOMEBREW if you don't want to use gettext and msgfmt
-# installed by Homebrew.
+# Define NO_HOMEBREW if you don't want to use gettext, libiconv and
+# msgfmt installed by Homebrew.
 #
 # Define HOMEBREW_PREFIX if you have Homebrew installed in a non-default
 # location on macOS or on Linux and want to use it.
 #
+# Define USE_HOMEBREW_LIBICONV to link against libiconv installed by
+# Homebrew, if present.
+#
 # Define NO_APPLE_COMMON_CRYPTO if you are building on Darwin/Mac OS X
 # and do not want to use Apple's CommonCrypto library.  This allows you
 # to provide your own OpenSSL library, for example from MacPorts.
@@ -1705,6 +1708,11 @@ endif
 ifeq ($(shell test -x $(HOMEBREW_PREFIX)/opt/gettext/msgfmt && echo y),y)
 	MSGFMT = $(HOMEBREW_PREFIX)/opt/gettext/msgfmt
 endif
+ifdef USE_HOMEBREW_LIBICONV
+ifeq ($(shell test -d $(HOMEBREW_PREFIX)/opt/libiconv && echo y),y)
+	ICONVDIR ?= $(HOMEBREW_PREFIX)/opt/libiconv
+endif
+endif
 endif
 endif
 

config.mak.uname

@@ -124,6 +124,7 @@ ifeq ($(uname_S),Darwin)
 	# - MacOS 10.0.* and MacOS 10.1.0 = Darwin 1.*
 	# - MacOS 10.x.* = Darwin (x+4).* for (1 <= x)
 	# i.e. "begins with [15678] and a dot" means "10.4.* or older".
+	DARWIN_MAJOR_VERSION = $(shell expr "$(uname_R)" : '\([0-9]*\)\.')
         ifeq ($(shell expr "$(uname_R)" : '[15678]\.'),2)
 		OLD_ICONV = UnfortunatelyYes
 		NO_APPLE_COMMON_CRYPTO = YesPlease
@@ -154,6 +155,9 @@ ifeq ($(uname_S),Darwin)
         else
 		HOMEBREW_PREFIX = /usr/local
         endif
+        ifeq ($(shell test "$(DARWIN_MAJOR_VERSION)" -ge 24 && echo 1),1)
+		USE_HOMEBREW_LIBICONV = UnfortunatelyYes
+        endif
 
 	# The builtin FSMonitor on MacOS builds upon Simple-IPC.  Both require
 	# Unix domain sockets and PThreads.