Test on macOS 12.2.1 against the master branch. #24397 doesn't fix it.

```
brew info boost
boost: stable 1.78.0 (bottled), HEAD
...
./configure --enable-external-signer
...
checking for boostlib >= 1.64.0 (106400)... yes
checking whether Boost.Process can be used... no
configure: error: External signing is not supported for this Boost version
```

It does work with boost 1.76 (there was no 1.77 release in home-brew):

```
brew install boost@1.76
...
LDFLAGS="-L/usr/local/opt/boost@1.76/lib" CPPFLAGS="-I/usr/local/opt/boost@1.76/include" ./configure --enable-external-signer
...
checking for boostlib >= 1.64.0 (106400)... yes
checking whether Boost.Process can be used... yes
...
Options used to compile and link:
  external signer = yes
```

There's nothing in the release notes about Boost.Process for either [1.77](https://www.boost.org/users/history/version_1_77_0.html) or [1.78](https://www.boost.org/users/history/version_1_78_0.html).

The Homebrew [formula changes](https://github.com/Homebrew/homebrew-core/commits/master/Formula/boost.rb) are nothing but a trivial version and hash bump.
{
    LOCK(cs_dir_locks);
    fs::path pathLockFile = directory / lockfile_name;
    
    if (dir_locks.count(fs::PathToString(pathLockFile))) {
        return true;
    }
    
    FILE* file = fsbridge::fopen(pathLockFile, "a");
    if (file) fclose(file);
    auto lock = std::make_unique<fsbridge::FileLock>(pathLockFile);
    if (!lock->TryLock()) {
        return error("Error while attempting to lock directory %s: %s", fs::PathToString(directory), lock->GetReason());
    }
    if (!probe_only) {
        
        dir_locks.emplace(fs::PathToString(pathLockFile), std::move(lock));
    }
    return true;
}
void UnlockDirectory(const fs::path& directory, const std::string& lockfile_name)