Check for presence of compiler -pthread flag.

OpenBSD / FreeBSD and some other OS's require the use of
cc -pthread to link threaded programs so have QEMU's
configure script check for the presence of the flag
and use it if so.

Signed-off-by: Brad Smith <brad@comstyle.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
