Compiling `--with-lto` results in `test_embed` failures on Darwin

Bug report

Bug description:

On Darwin, configure.ac:1998 sets:

LTOFLAGS="-flto=thin -Wl,-export_dynamic -Wl,-object_path_lto,\"\$@\".lto"

Which gets expanded too early in Makefile.pre.in:130 into "".lto (notice the :=):

PY_CORE_EXE_LDFLAGS:= $(if $(CONFIGURE_EXE_LDFLAGS), $(CONFIGURE_EXE_LDFLAGS) $(PY_LDFLAGS_NODIST), $(PY_CORE_LDFLAGS))

Linking results in:

gcc    -flto=thin -Wl,-export_dynamic -Wl,-object_path_lto,"".lto -g  ...

Which happens to link Programs/_testembed as the interpreter, so that launching it results in:

$ ./Programs/_testembed           
Could not find platform independent libraries <prefix>
Could not find platform dependent libraries <exec_prefix>
WARN: Could not find the standard library directory! sys.prefix is set to /usr/local, is this correct?
Python 3.16.0a0 (heads/main:4f39efc47de, Jul  7 2026, 17:34:29) [Clang 21.0.0 (clang-2100.1.1.101)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 

And related tests in the test_embed suite fail:

$ ./python.exe -m test -v test_embed
== CPython 3.16.0a0 (heads/main:4f39efc47de, Jul 7 2026, 17:34:29) [Clang 21.0.0 (clang-2100.1.1.101)]
== macOS-26.5.1-arm64-arm-64bit-Mach-O little-endian
== Python build: release ThinLTO
== cwd: ***
== CPU count: 16
== encodings: locale=UTF-8 FS=utf-8
== resources: all test resources are disabled, use -u option to unskip tests

Using random seed: 2037373937
0:00:00 load avg: 2.73 mem: 31.0 MiB Run 1 test sequentially in a single process
0:00:00 load avg: 2.73 mem: 31.0 MiB [1/1] test_embed
test_audit (test.test_embed.AuditingTests.test_audit) ... --- ['***/cpython/Programs/_testembed', 'test_audit'] failed ---
stdout:

stderr:
Could not find platform independent libraries <prefix>
Could not find platform dependent libraries <exec_prefix>
WARN: Could not find the standard library directory! sys.prefix is set to /usr/local, is this correct?
***/Programs/_testembed: can't open file '***/test_audit': [Errno 2] No such file or directory

------
FAIL

It seems to me this was introduced by 4e45c9c.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-153532

Start by comparing configure.ac:1998 with Makefile.pre.in:130, focusing on how LTOFLAGS is expanded during the Darwin build. Reproduce the ThinLTO build and run ./python.exe -m test -v test_embed. Done means _testembed launches correctly and the test_embed suite passes; linked PR gh-153532 indicates work may already be underway.