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.
if (key[0] == '/')
            key[0] = '-';
#endif
        if (key[0] != '-') {
            if (!m_accept_any_command && m_command.empty()) {
                
                std::optional<unsigned int> flags = GetArgFlags(key);
                if (!flags || !(*flags & ArgsManager::COMMAND)) {
                    error = strprintf("Invalid command '%s'", argv[i]);
                    return false;
                }
            }
            m_command.push_back(key);
            while (++i < argc) {
                
                m_command.push_back(argv[i]);
            }
            break;
        }
        
        if (key.length() > 1 && key[1] == '-')
            key.erase(0, 1);
        
        key.erase(0, 1);
        KeyInfo keyinfo = InterpretKey(key);
        std::optional<unsigned int> flags = GetArgFlags('-' + keyinfo.name);
        
        
        
        if (!flags || !keyinfo.section.empty()) {