Fallback to srand/rand if srandom/random are missing

[scribam]

From #156

1. srandom and random are POSIX extensions not present on non-POSIX
   platforms, replace them with srand and rand respectively. They
   might produce different results, but that shouldn't matter in the
   context of the caller, which is the non-secure version

uwp and win32 already use these functions, we can unify all platforms with these changes

vitasdk (a sdk for playstation vita) has the same patch (https://github.com/vitasdk/packages/blob/master/libzip/random.patch) in order to build libzip and I would like to remove it :)

[dillof]

IIRC, some systems have deprecation warning for rand(). I'd therefore prefer to have cmake check for the existence of random() and only fall back to rand() if it's missing.

Also, while reading the diff, I just noticed that we never set seeded to true. Could you please add this, so we avoid conflicts?