configure.ac: Fix pthread linking on FreeBSD.

The configure script checks for the existence of pthread_sigmask.
However, on FreeBSD, libc contains no-op stubs for many of the
pthread_* functions.  As a result, the AC_SEARCH_LIBS macro returns
"none required".

As an alternative to checking pthread_sigmask, a solution is to check
pthread_create.

Signed-off-by: Kevin Lo <kevlo@FreeBSD.org>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>

configure.ac

@@ -81,7 +81,7 @@ AC_SUBST([LT_AGE])
 AC_SEARCH_LIBS([pow], [m])
 AC_SEARCH_LIBS([clock_gettime], [rt])
 AC_SEARCH_LIBS([timer_create], [rt])
-AC_SEARCH_LIBS([pthread_sigmask], [pthread])
+AC_SEARCH_LIBS([pthread_create], [pthread])
 AC_FUNC_STRERROR_R
 
 OVS_CHECK_ESX