Properly detect POSIX threads on OpenBSD.

The following diff for VLC 1.0.0rc3 corrects the detection of POSIX threads
on OpenBSD.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
(cherry picked from commit bb1d1eb5)

configure.ac

@@ -837,6 +837,9 @@ fi
 if test "${THREAD_LIB}" = "error"; then
   AC_CHECK_LIB(c_r,main,THREAD_LIB="-lc_r")
 fi
+if test "${THREAD_LIB}" = "error"; then
+  AC_CHECK_LIB(pthread,main,THREAD_LIB="-pthread")
+fi
 if test "${THREAD_LIB}" = "error"; then
   AC_CHECK_FUNCS(pthread_mutex_lock)
   THREAD_LIB=""