Removing LIBPTHREAD/LIBTHR build options

[Kyle Evans]

Hi,

I've staged a review to remove the LIBPTHREAD/LIBTHR build options:
https://reviews.freebsd.org/D28263

WITHOUT_LIBTHR (and WITHOUT_LIBPTHREAD by proxy) was broken over five
years ago, and the situation has only deteriorated since. It's hard to
see the value in building a system without threading support entirely
and no one has fixed it in the meantime, so I think it's time to stop
pretending we can offer it.

Comments?

Thanks,

Kyle Evans