Changes to remove the pthread_np.h include file from WiredTiger's build, and use the x86intrin.h include file to configure the vectorization instructions.

@sueloverso, would you please confirm this build works on your laptop, and if it does, merge it?

@agorrod, @michaelcahill: I added pthread_np.h to the build in commit 562ae1f, to support the use of pthread_timedjoin_np, as far as I can tell, we no longer use any pthread_XXX_np functions.