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>

configure

@@ -1856,7 +1856,7 @@ fi
 
 ##########################################
 # pthread probe
-PTHREADLIBS_LIST="-lpthread -lpthreadGC2"
+PTHREADLIBS_LIST="-pthread -lpthread -lpthreadGC2"
 
 pthread=no
 cat > $TMPC << EOF