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.
{
#ifdef ENABLE_EXTERNAL_SIGNER
    namespace bp = boost::process;
    UniValue result_json;
    bp::opstream stdin_stream;
    bp::ipstream stdout_stream;
    bp::ipstream stderr_stream;
    if (str_command.empty()) return UniValue::VNULL;
    bp::child c(
        str_command,
        bp::std_out > stdout_stream,
        bp::std_err > stderr_stream,
        bp::std_in < stdin_stream
    );
    if (!str_std_in.empty()) {
        stdin_stream << str_std_in << std::endl;
    }
    stdin_stream.pipe().close();
    std::string result;
    std::string error;
    std::getline(stdout_stream, result);
    std::getline(stderr_stream, error);
    c.wait();
    const int n_error = c.exit_code();
    if (n_error) throw std::runtime_error(strprintf("RunCommandParseJSON error: process(%s) returned %d: %s\n", str_command, n_error, error));
    if (!result_json.read(result)) throw std::runtime_error("Unable to parse JSON: " + result);
    return result_json;
#else