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.
error = strprintf("Invalid parameter %s", argv[i]);
            return false;
        }
        std::optional<util::SettingsValue> value = InterpretValue(keyinfo, val, *flags, error);
        if (!value) return false;
        m_settings.command_line_options[keyinfo.name].push_back(*value);
    }
    
    if (auto* includes = util::FindKey(m_settings.command_line_options, "includeconf")) {
        const util::SettingsSpan values{*includes};
        
        if (!values.empty()) {
            error = "-includeconf cannot be used from commandline; -includeconf=" + values.begin()->write();
            return false; 
        }
    }
    return true;
}
std::optional<unsigned int> ArgsManager::GetArgFlags(const std::string& name) const
{
    LOCK(cs_args);
    for (const auto& arg_map : m_available_args) {
        const auto search = arg_map.second.find(name);
        if (search != arg_map.second.end()) {
            return search->second.m_flags;
        }
    }
    return std::nullopt;
}
fs::path ArgsManager::GetPathArg(std::string arg, const fs::path& default_value) const