asprintf: globally define _GNU_SOURCE

asprintf: globally define _GNU_SOURCE

And also provide a replacement for asprintf if it is not available. I
would prefer to not use _GNU_SOURCE everywhere but it seems cleaner than
putting it at the top of random files. Ideally, I should wrap every
function in a separate file to just use _GNU_SOURCE for those
functions. There are `asprintf()` and `setres[ug]id()`.