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);
        m_settings.ro_config.clear();
        m_config_sections.clear();
    }
    const std::string confPath = GetArg("-conf", BITCOIN_CONF_FILENAME);
    std::ifstream stream{GetConfigFile(confPath)};
    
    if (IsArgSet("-conf") && !stream.good()) {
        error = strprintf("specified config file \"%s\" could not be opened.", confPath);
        return false;
    }
    
    if (stream.good()) {
        if (!ReadConfigStream(stream, confPath, error, ignore_invalid_keys)) {
            return false;
        }
        
        
        bool use_conf_file{true};
        {
            LOCK(cs_args);
            if (auto* includes = util::FindKey(m_settings.command_line_options, "includeconf")) {