osxkeychain: define build targets in the top-level Makefile.
The fix for git-credential-osxkeychain in 4580bcd235 (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 864f55e190 because several macOS-specific refinements were applied to the top-level Makefile and config.mak.uname, such as: - 363837afe7 (macOS: make Homebrew use configurable, 2025-12-24) - cee341e9dd (macOS: use iconv from Homebrew if needed and present, 2025-12-24) - d281241518 (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>