autotools.py: set lt_cv_apple_cc_single_mod=yes (#48671)

Since macOS 15 `ld -single_module` warns with a deprecation message,
which makes configure scripts believe the flag is unsupported. That
in turn triggers a code path where `archive_cmds` is set to

```
$CC -r -keep_private_externs -nostdlib ... -dynamiclib
```

instead of just

```
$CC -dynamiclib ...
```

This code path was meant to trigger only on ancient macOS <= 14.4 where
libtool had to add `-single_module`, which is the default since macos
14.4, and is now apparently deprecated because the flag is a no-op for
more than 15 years.

The wrong `archive_cmds` causes actual problems combined with a bug in
OpenMPI's compiler wrapper (`CC=mpicc`), which appends `-rpath` flags,
which cause an error when combined with the `-r` flag added by the
autotools.

Spack's compiler wrapper doesn't do this, but it's likely there are
other compiler wrappers out there that are not aware that `-r` and
`-rpath` cannot be combined.

The fix is to change defaults: `lt_cv_apple_cc_single_mod=yes`.

lib/spack/spack/build_systems/autotools.py

@@ -356,6 +356,13 @@ def _do_patch_libtool_configure(self) -> None:
             )
             # Support Libtool 2.4.2 and older:
             x.filter(regex=r'^(\s*test \$p = "-R")(; then\s*)$', repl=r'\1 || test x-l = x"$p"\2')
+            # Configure scripts generated with libtool < 2.5.4 have a faulty test for the
+            # -single_module linker flag. A deprecation warning makes it think the default is
+            # -multi_module, triggering it to use problematic linker flags (such as ld -r). The
+            # linker default is `-single_module` from (ancient) macOS 10.4, so override by setting
+            # `lt_cv_apple_cc_single_mod=yes`. See the fix in libtool commit
+            # 82f7f52123e4e7e50721049f7fa6f9b870e09c9d.
+            x.filter("lt_cv_apple_cc_single_mod=no", "lt_cv_apple_cc_single_mod=yes", string=True)
 
     @spack.phase_callbacks.run_after("configure")
     def _do_patch_libtool(self) -> None: