Merging r155535:

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r155535 | lattner | 2012-04-24 23:12:24 -0700 (Tue, 24 Apr 2012) | 5 lines

OpenBSD: Remove incorrect -pthread preprocessor define _POSIX_THREADS and replace
with _REENTRANT. Also remove undef _POSIX_THREADS in phread.h.

Patch by Brad Smith!

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llvm-svn: 155573