Minor style cleanup of thread test script.

config/acx_pthread.m4

@@ -168,9 +168,9 @@ if test "x$acx_pthread_ok" = xyes; then
 
         AC_MSG_CHECKING([if more special flags are required for pthreads])
         flag=no
-# We handle this ourselves in PostgreSQL
+# We always add these in PostgreSQL
 #       case "${host_cpu}-${host_os}" in
-#                *-aix* | *-freebsd*)     flag="-D_THREAD_SAFE";;
+#               *-aix* | *-freebsd* | *-darwin*) flag="-D_THREAD_SAFE";;
 #               *solaris* | *-osf* | *-hpux*) flag="-D_REENTRANT";;
 #       esac
         AC_MSG_RESULT(${flag})