fix pthread_detach check

Upstream: https://bugs.ntp.org/show_bug.cgi?id=3926

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
---
 sntp/m4/openldap-thread-check.m4 |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

Index: ntp-4.2.8p17/sntp/m4/openldap-thread-check.m4
===================================================================
--- ntp-4.2.8p17.orig/sntp/m4/openldap-thread-check.m4
+++ ntp-4.2.8p17/sntp/m4/openldap-thread-check.m4
@@ -262,10 +262,8 @@ pthread_rwlock_t rwlock;
 				dnl save the flags
 				AC_LINK_IFELSE([AC_LANG_PROGRAM([[
 #include <pthread.h>
-#ifndef NULL
-#define NULL (void*)0
-#endif
-]], [[pthread_detach(NULL);]])],[ol_cv_func_pthread_detach=yes],[ol_cv_func_pthread_detach=no])
+pthread_t thread;
+]], [[pthread_detach(thread);]])],[ol_cv_func_pthread_detach=yes],[ol_cv_func_pthread_detach=no])
 			])

 			if test $ol_cv_func_pthread_detach = no ; then