Delete dependency on config.h

Summary:
Since llvm/Config/config.h is not available on standalone builds,
use __USE_POSIX instead of HAVE_PTHREAD_H and get rid of the include.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: lebedev.ri, krytarowski, ilya-biryukov, ioeric, jkorous, arphaman, jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D53935

llvm-svn: 345729