The new code is faster and reuses the previous state in case of
multiple calls.

The previous code could easily end up in near-infinite loops,
if the difference between two clock() calls never was larger than
1.

This makes fate-parseutils finish in finite time when run in wine,
if CryptGenRandom isn't available (which e.g. isn't available if
targeting Windows RT/metro).

Patch originally by Michael Niedermayer but with some modifications
by Martin Storsjö.
Signed-off-by: Martin Storsjö <martin@martin.st>