Remove the -pthread option (in FreeBSD versions 500016 and greater) as

Remove the -pthread option (in FreeBSD versions 500016 and greater) as
threatened over 2 years ago.

Why?  -pthread was a hack to prevent linking to both libc and libc_r
and became unecessary when libc_r became free of libc.  Now that we
have multiple thread libraries from which to choose, it is more confusing
because you can't link to more than one threads library at a time.
Things like autoconf and libtool sometimes detect -pthread and
also -lc_r, and in conjunction with ports usage of ${PTHREAD_LIBS},
really wacky things ensue when PTHREAD_LIBS is set to another
threads library.  This might not be so bad if the build broke
when this happens, but it doesn't and you don't know it until
funny things happen when you run the application (or use an
affected library).

Reviewed by:    obrien