4MB per thread may be too big on some systems, set to 2MB for now

include/ldap_pvt_thread.h

@@ -43,7 +43,7 @@ ldap_pvt_thread_set_concurrency LDAP_P(( int ));
 
 #ifndef LDAP_PVT_THREAD_STACK_SIZE
 	/* larger stack */
-#define LDAP_PVT_THREAD_STACK_SIZE	(4*1024*1024)
+#define LDAP_PVT_THREAD_STACK_SIZE	(2*1024*1024)
 #endif
 
 LDAP_F( int )