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_args);
    std::vector<std::pair<std::string, std::string>> options;
    if (!GetConfigOptions(stream, filepath, error, options, m_config_sections)) {
        return false;
    }
    for (const std::pair<std::string, std::string>& option : options) {
        KeyInfo key = InterpretKey(option.first);
        std::optional<unsigned int> flags = GetArgFlags('-' + key.name);
        if (flags) {
            std::optional<util::SettingsValue> value = InterpretValue(key, option.second, *flags, error);
            if (!value) {
                return false;
            }
            m_settings.ro_config[key.section][key.name].push_back(*value);
        } else {
            if (ignore_invalid_keys) {
                LogPrintf("Ignoring unknown configuration value %s\n", option.first);
            } else {
                error = strprintf("Invalid configuration value %s", option.first);
                return false;
            }
        }
    }
    return true;
}
bool ArgsManager::ReadConfigFiles(std::string& error, bool ignore_invalid_keys)