osxkeychain: define build targets in the top-level Makefile.

The fix for git-credential-osxkeychain in 4580bcd (osxkeychain: avoid
incorrectly skipping store operation, 2025-11-14) introduced linkage
with libgit.a, and its Makefile was adjusted accordingly. However, the
build fails as of 864f55e because several macOS-specific refinements
were applied to the top-level Makefile and config.mak.uname, such as:

  - 363837a (macOS: make Homebrew use configurable, 2025-12-24)
  - cee341e (macOS: use iconv from Homebrew if needed and present,
    2025-12-24)
  - d281241 (utf8.c: enable workaround for iconv under macOS 14/15,
    2026-01-12)

Since libgit.a and its corresponding header files depend on many flags
defined in the top-level Makefile, these flags must be consistently
defined when building git-credential-osxkeychain. Continuing to manually
adjust the git-credential-osxkeychain Makefile is cumbersome and
fragile.

Define the build targets for git-credential-osxkeychain in the top-level
Makefile and modify its local Makefile to simply rely on those targets.

Helped-by: Junio C Hamano <gitster@pobox.com>
Reported-by: D. Ben Knoble <ben.knoble@gmail.com>
Helped-by: Kristoffer Haugsbakk <kristofferhaugsbakk@fastmail.com>
Signed-off-by: Koji Nakamaru <koji.nakamaru@gree.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: KojiNakamaru

Makefile

@@ -2874,6 +2874,10 @@ objects: $(OBJECTS)
 dep_files := $(foreach f,$(OBJECTS),$(dir $f).depend/$(notdir $f).d)
 dep_dirs := $(addsuffix .depend,$(sort $(dir $(OBJECTS))))
 
+ifeq ($(uname_S),Darwin)
+	dep_dirs += $(addsuffix .depend,$(sort $(dir contrib/credential/osxkeychain/git-credential-osxkeychain.o)))
+endif
+
 ifeq ($(COMPUTE_HEADER_DEPENDENCIES),yes)
 $(dep_dirs):
 	@mkdir -p $@
@@ -4058,3 +4062,20 @@ $(LIBGIT_HIDDEN_EXPORT): $(LIBGIT_PARTIAL_EXPORT)
 
 contrib/libgit-sys/libgitpub.a: $(LIBGIT_HIDDEN_EXPORT)
 	$(AR) $(ARFLAGS) $@ $^
+
+contrib/credential/osxkeychain/git-credential-osxkeychain: contrib/credential/osxkeychain/git-credential-osxkeychain.o $(LIB_FILE) GIT-LDFLAGS
+	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) \
+		$(filter %.o,$^) $(LIB_FILE) $(EXTLIBS) -framework Security -framework CoreFoundation
+
+contrib/credential/osxkeychain/git-credential-osxkeychain.o: contrib/credential/osxkeychain/git-credential-osxkeychain.c GIT-CFLAGS
+	$(QUIET_LINK)$(CC) -o $@ -c $(dep_args) $(compdb_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
+
+install-git-credential-osxkeychain: contrib/credential/osxkeychain/git-credential-osxkeychain
+	$(INSTALL) -d -m 755 '$(DESTDIR_SQ)$(gitexec_instdir_SQ)'
+	$(INSTALL) $(INSTALL_STRIP) $< '$(DESTDIR_SQ)$(gitexec_instdir_SQ)'
+
+.PHONY: clean-git-credential-osxkeychain
+clean-git-credential-osxkeychain:
+	$(RM) \
+		contrib/credential/osxkeychain/git-credential-osxkeychain \
+		contrib/credential/osxkeychain/git-credential-osxkeychain.o

contrib/credential/osxkeychain/Makefile

@@ -1,66 +1,13 @@
 # The default target of this Makefile is...
 all:: git-credential-osxkeychain
 
-include ../../../config.mak.uname
--include ../../../config.mak.autogen
--include ../../../config.mak
+git-credential-osxkeychain:
+	$(MAKE) -C ../../.. contrib/credential/osxkeychain/git-credential-osxkeychain
 
-ifdef ZLIB_NG
-	BASIC_CFLAGS += -DHAVE_ZLIB_NG
-        ifdef ZLIB_NG_PATH
-		BASIC_CFLAGS += -I$(ZLIB_NG_PATH)/include
-		EXTLIBS += $(call libpath_template,$(ZLIB_NG_PATH)/$(lib))
-        endif
-	EXTLIBS += -lz-ng
-else
-        ifdef ZLIB_PATH
-		BASIC_CFLAGS += -I$(ZLIB_PATH)/include
-		EXTLIBS += $(call libpath_template,$(ZLIB_PATH)/$(lib))
-        endif
-	EXTLIBS += -lz
-endif
-ifndef NO_ICONV
-        ifdef NEEDS_LIBICONV
-                ifdef ICONVDIR
-			BASIC_CFLAGS += -I$(ICONVDIR)/include
-			ICONV_LINK = $(call libpath_template,$(ICONVDIR)/$(lib))
-                else
-			ICONV_LINK =
-                endif
-                ifdef NEEDS_LIBINTL_BEFORE_LIBICONV
-			ICONV_LINK += -lintl
-                endif
-		EXTLIBS += $(ICONV_LINK) -liconv
-        endif
-endif
-ifndef LIBC_CONTAINS_LIBINTL
-	EXTLIBS += -lintl
-endif
-
-prefix ?= /usr/local
-gitexecdir ?= $(prefix)/libexec/git-core
-
-CC ?= gcc
-CFLAGS ?= -g -O2 -Wall -I../../.. $(BASIC_CFLAGS)
-LDFLAGS ?= $(BASIC_LDFLAGS) $(EXTLIBS)
-INSTALL ?= install
-RM ?= rm -f
-
-%.o: %.c
-	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ -c $<
-
-git-credential-osxkeychain: git-credential-osxkeychain.o ../../../libgit.a
-	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS) \
-		-framework Security -framework CoreFoundation
-
-install: git-credential-osxkeychain
-	$(INSTALL) -d -m 755 $(DESTDIR)$(gitexecdir)
-	$(INSTALL) -m 755 $< $(DESTDIR)$(gitexecdir)
-
-../../../libgit.a:
-	cd ../../..; make libgit.a
+install:
+	$(MAKE) -C ../../.. install-git-credential-osxkeychain
 
 clean:
-	$(RM) git-credential-osxkeychain git-credential-osxkeychain.o
+	$(MAKE) -C ../../.. clean-git-credential-osxkeychain
 
-.PHONY: all install clean
+.PHONY: all git-credential-osxkeychain install clean