Target an even older version of macOS

[ISSOtm]

We can, so let's not ask if we should!

Okay, so, turns out that Apple's linker just... throws an assertion failure:

ld: warning: support for macOS with 10.4 minimum deployment target is deprecated and will be removed in future release
0  0x104707474  __assert_rtn + 140
1  0x104714158  ld::passes::stubs::arm::StubHelperAtom::StubHelperAtom(ld::passes::stubs::Pass&, ld::Atom const&, ld::Atom const*, bool) (.cold.1) + 0
2  0x10465ea48  ld::passes::stubs::x86_64::classic::StubHelperAtom::~StubHelperAtom() + 0
3  0x10465bcd4  ld::passes::stubs::Pass::makeStub(ld::Atom const&, bool) + 4740
4  0x10465cef4  ld::passes::stubs::Pass::process(ld::Internal&) + 1396
5  0x10465d42c  ld::passes::stubs::doPass(Options const&, ld::Internal&) + 100
6  0x1044ff028  main + 664
A linker snapshot was created at:
    /tmp/script-x86_64.out-2026-04-13-230001.ld-snapshot
ld: Assertion failed: (targetAtom != NULL), function Fixup, file ld.hpp, line 1045.

So we have to use LLD instead, which needs to be installed explicitly. I'm doing it as a bodge in the initial version of this PR, as I want to use the “toolset” mechanism from #1906 to do that properly.

[Rangi42]

Even if this is meant as a temporary solution prior to #1906, it's still a niche enough use case that it should be clearly commented.

Suggested change

	brew install lld
	brew install lld # Apple's linker cannot support Mac OS X 10.4, so we use `lld`

[Rangi42]

There's never been a problem with Apple's ld on 10.9 or above, and supporting 10.9 Mavericks from 2013 is already niche enough -- so any changes that are needed for 10.4 Tiger from 2005, over two decades ago, really ought to be explained.

Suggested change

	# The `-mmacosx-version-min=10.4` flag ensures that the binary only uses APIs available on Mac OS X 10.4 Tiger.
	# The `-arch` flags build a "fat binary" that works on both Apple architectures:
	# older Intel x64 Macs and newer ARM "Apple Silicon" ones.
	set(secret_sauce -mmacosx-version-min=10.9 "SHELL:-arch x86_64" "SHELL:-arch arm64") # Avoid `-arch` being dedup'd.
	set(secret_sauce -mmacosx-version-min=10.4 "SHELL:-arch x86_64" "SHELL:-arch arm64") # Avoid `-arch` being dedup'd.
	add_compile_options(${secret_sauce})
	add_link_options(${secret_sauce})
	add_link_options(${secret_sauce} -fuse-ld=lld) # Apple's linker just crashes.
	set(PNG_HARDWARE_OPTIMIZATIONS OFF) # These do not play well with a dual-arch build.
	# The `-mmacosx-version-min=10.4` flag ensures that the binary only uses APIs available on Mac OS X 10.4 Tiger.
	# The `-arch` flags build a "fat binary" that works on both Apple architectures:
	# older Intel x64 Macs and newer ARM "Apple Silicon" ones.
	set(secret_sauce -mmacosx-version-min=10.4 "SHELL:-arch x86_64" "SHELL:-arch arm64") # Avoid `-arch` being dedup'd.
	add_compile_options(${secret_sauce})
	# The `-fuse-ld=lld` flag specifies an alternative to Apple's linker, because recent versions warn that
	# "support for macOS with 10.4 minimum deployment target is deprecated and will be removed in future release",
	# and may crash with an assertion failure.
	add_link_options(${secret_sauce} -fuse-ld=lld)
	set(PNG_HARDWARE_OPTIMIZATIONS OFF) # These do not play well with a dual-arch build.