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.
{
    KeyInfo result;
    
    size_t option_index = key.find('.');
    if (option_index != std::string::npos) {
        result.section = key.substr(0, option_index);
        key.erase(0, option_index + 1);
    }
    if (key.substr(0, 2) == "no") {
        key.erase(0, 2);
        result.negated = true;
    }
    result.name = key;
    return result;
}
/**
 * Interpret settings value based on registered flags.
 *
 * @param[in]   key      key information to know if key was negated
 * @param[in]   value    string value of setting to be parsed
 * @param[in]   flags    ArgsManager registered argument flags
 * @param[out]  error    Error description if settings value is not valid
 *
 * @return parsed settings value if it is valid, otherwise nullopt accompanied
 * by a descriptive error string
 */
static std::optional<util::SettingsValue> InterpretValue(const KeyInfo& key, const std::string& value,