pthread_yield needs GNU_SOURCE (missing in the test here), but pthread_yield
is both deprecated and also not actually used (even conditionally!) in zthread,
so let's just drop the test.

(zthread uses sched_yield conditionally, however.)
--- a/share/pthread.m4
+++ b/share/pthread.m4
@@ -128,14 +128,6 @@ pthread_explicit="no"
       AC_DEFINE(HAVE_SCHED_YIELD,,[Defined if sched_yield() is available]) ],  
     [ AC_MSG_RESULT(no) ])
 
-  dnl Check for pthread_yield
-  AC_MSG_CHECKING(for pthread_yield);
-  AC_TRY_LINK([#include <pthread.h>],
-    [ pthread_yield(); ], 
-    [ AC_MSG_RESULT(yes)
-      AC_DEFINE(HAVE_PTHREAD_YIELD,,[Defined if pthread_yield() is available]) ],  
-    [ AC_MSG_RESULT(no) ])
-
   dnl Check for pthread_key_create
   AC_MSG_CHECKING(for pthread_key_create)
   AC_TRY_LINK([#include <pthread.h>],