[x265] Fix signal handler

[SF Markus Elfring]

> Sorry, I'm not seeing the point.  If the CLI app is calling exit(), why do I
> care if the SIGINT handler can set a flag or not? 

I try to point out that there are risks involved with the current
implementation. The calling of a sync-signal-unsafe function by a signal handler
has got the general potential to result in a deadlock of your program, hasn't it?
It is a matter if you design also this source code place for software correctness.

Would you like to consider the other approaches for such an use case?
- self-pipe trick
  http://cr.yp.to/docs/selfpipe.html

- sigwait
  http://pubs.opengroup.org/onlinepubs/9699919799/functions/sigwait.html

Regards,
Markus