Re: [PATCH v2] Makefile: link osxkeychain & support universal Rust

> From: Shnatu <snatu@google.com>
> 
> When Rust is enabled, ensure that the git-credential-osxkeychain
> helper is linked with the necessary Rust libraries.
> 
> Also, introduce native support for macOS Universal Binaries
> (multi-architecture builds) in the Git build system by allowing
> the user to specify a list of target triples in the RUST_TARGETS
> environment variable.

> To implement this cleanly without complex shell scripting in recipes:
>   1. We introduce a declarative Make pattern rule (target/%/...) to
>      compile each target-specific library slice (e.g.,
>      target/aarch64-apple-darwin/...).
>   2. We update the $(RUST_LIB) recipe to depend on the list of
>      compiled target-specific member libraries ($(RUST_MEMBER_LIBS)).
>   3. On macOS, if multiple targets are specified, we use lipo to
>      combine them into a single Universal static library at
>      target/release/libgitcore.a.
>   4. If only one target is specified, we copy it to the standard
>      path.
>   5. We enforce that building for multiple targets requires macOS
>      (as lipo is only available there), raising a clear make error
>      on other platforms.
> 
> This is a highly elegant and native Makefile solution that avoids
> complex shell scripting in recipes and fully supports macOS Universal
> Binaries.

> diff --git a/Makefile b/Makefile
> index 1f3f099f5c..8d49ecc897 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -3019,11 +3030,33 @@ scalar$X: scalar.o GIT-LDFLAGS $(GITLIBS)
>  $(LIB_FILE): $(LIB_OBJS)
>  	$(QUIET_AR)$(RM) $@ && $(AR) $(ARFLAGS) $@ $^
>  
> +ifndef NO_RUST
> +ifeq ($(RUST_TARGETS),)
>  $(RUST_LIB): Cargo.toml $(RUST_SOURCES) $(LIB_FILE)
>  	$(QUIET_CARGO)cargo build $(CARGO_ARGS)
> +else
> +ifneq ($(words $(RUST_TARGETS)),1)
> +ifneq ($(uname_S),Darwin)
> +$(error Building universal Rust libraries requires macOS (lipo is not available on $(uname_S)))
> +endif
> +endif
> +
> +RUST_MEMBER_LIBS = $(foreach target,$(RUST_TARGETS),target/$(target)/$(RUST_BUILD_CONFIG)/$(RUST_LIB_NAME))
> +$(RUST_MEMBER_LIBS): target/%/$(RUST_BUILD_CONFIG)/$(RUST_LIB_NAME): Cargo.toml $(RUST_SOURCES) $(LIB_FILE)
> 
> +	$(QUIET_CARGO)cargo build $(CARGO_ARGS) --target $*
> +
> +$(RUST_LIB): $(RUST_MEMBER_LIBS)
> +	$(QUIET_GEN)\
> +	if [ $(words $(RUST_TARGETS)) -gt 1 ]; then \
> +		lipo -create $^ -output $@; \