[AdemiseG]

Follow-up to #34924, as discussed there: #34924 (comment)

-ld_classic is gated on cc_version_gte(cfg, 15, 0) with no upper bound, so every clang from 15 upward is passed it. Apple has since removed the classic linker, so on current Xcode every macOS link emits:

ld: warning: -ld_classic is no longer supported and will be ignored

The flag was added for Xcode 15.3 for a real reason, so it cannot simply be dropped. This probes for it at configure time instead, following the check_librt pattern in cxx_checks.py. The complication is that the flag only produces a warning, not an error, so a plain link test succeeds on both old and new linkers; the check links a trivial program with -fatal_warnings to turn that warning into a failure.

Linkers that still accept -ld_classic are unaffected and keep the flag.

Testing

macOS 27.0.1 (26A434), Apple Silicon arm64, Apple clang 21.0.0 (clang-2100.3.34.2), ld PROJECT:ld-27037.1.

Configure reports the new check alongside the existing ones:

CXX Compiler                             : clang++ 21.0.0
Checking for -ld_classic linker support  : unavailable

and -ld_classic is absent from the stored LINKFLAGS, with -dead_strip retained:

LINKFLAGS = ['-Wl,-dead_strip', '-pthread', '-fno-builtin']

./waf copter links with no linker warnings, and the resulting binary boots and runs the SITL sim loop.

To check that the probe discriminates rather than always failing, I re-ran it against a flag this linker does accept (-dead_strip): it reports available and the flag is kept. That is the Xcode 15.3 path, exercised by proxy -- I do not have a 15.3 toolchain to test directly, so confirmation from someone who does would be welcome.

Note on the build: master alone does not link on this toolchain until #34924 lands, so the build and run above were done with #34924 merged in locally. This branch itself contains only the two waf files.

[github-actions]

Global Size Summary (Total Flash delta in bytes)

Built from 05ae33c rebased onto 6743a33

Board	Periph	tracker	blimp	bootloader	copter	heli	iofw	plane	rover	sub
DrotekP3Pro	_	_	_	*	_	_	_	_	_	_
Durandal	_	*	*	_	*	*	_	*	*	*
Hitec-Airspeed	*	_	_	_	_	_	_	_	_	_
KakuteF7	_	*	*	_	*	*	_	*	*	*
MambaH743v4	_	*	*	_	*	*	_	*	*	*
MatekF405-CAN	_	_	_	*	_	_	_	_	_	_
MatekF405	_	*	*	_	*	*	_	*	*	*
MatekH743-bdshot	_	*	*	_	*	*	_	*	*	*
Pixhawk1-1M	_	*	*	_	*	*	_	*	*	*
disco	_	*	*	_	*	*	_	*	*	*
f103-GPS	*	_	_	_	_	_	_	_	_	_

[AdemiseG]

CI note: the one red check, autotest (sitltest-quadplane-tests1c), is a pre-existing flake and not caused by this PR.

It fails in QAUTOTUNE with NotAchievedException('autotune did not succeed'), after the vehicle reports AutoTune: Failed to level, please tune manually. The same job failed on master at 3cd7c7d (27 Oct) and 3d06638 (26 Oct).

This change also cannot reach that test: check_ld_classic() returns early unless DEST_OS == 'darwin', and ld_classic appears nowhere in that job's log. Happy to rebase if you'd like a clean run.