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`.
044d1b12bb