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 (fflush(file) != 0) { 
        LogPrintf("%s: fflush failed: %d\n", __func__, errno);
        return false;
    }
#ifdef WIN32
    HANDLE hFile = (HANDLE)_get_osfhandle(_fileno(file));
    if (FlushFileBuffers(hFile) == 0) {
        LogPrintf("%s: FlushFileBuffers failed: %d\n", __func__, GetLastError());
        return false;
    }
#elif defined(MAC_OSX) && defined(F_FULLFSYNC)
    if (fcntl(fileno(file), F_FULLFSYNC, 0) == -1) { 
        LogPrintf("%s: fcntl F_FULLFSYNC failed: %d\n", __func__, errno);
        return false;
    }
#elif HAVE_FDATASYNC
    if (fdatasync(fileno(file)) != 0 && errno != EINVAL) { 
        LogPrintf("%s: fdatasync failed: %d\n", __func__, errno);
        return false;
    }
#else
    if (fsync(fileno(file)) != 0 && errno != EINVAL) {
        LogPrintf("%s: fsync failed: %d\n", __func__, errno);
        return false;
    }
#endif
    return true;
}
void DirectoryCommit(const fs::path &dirname)