main: Use mallinfo2 when available at build time

main: Use mallinfo2 when available at build time

`mallinfo` has been deprecated in favor of `mallinfo2`:
```
The fields of the mallinfo structure that is returned by the
older mallinfo() function are typed as int.  However, because
some internal bookkeeping values may be of type long, the
reported values may wrap around zero and thus be inaccurate.
```

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1786>