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.
{
    BOOST_CHECK(true);
}
#ifdef ENABLE_EXTERNAL_SIGNER
BOOST_AUTO_TEST_CASE(run_command)
{
    {
        const UniValue result = RunCommandParseJSON("");
        BOOST_CHECK(result.isNull());
    }
    {
#ifdef WIN32
        const UniValue result = RunCommandParseJSON("cmd.exe /c echo {\"success\": true}");
#else
        const UniValue result = RunCommandParseJSON("echo \"{\"success\": true}\"");
#endif
        BOOST_CHECK(result.isObject());
        const UniValue& success = find_value(result, "success");
        BOOST_CHECK(!success.isNull());
        BOOST_CHECK_EQUAL(success.getBool(), true);