diff --git a/config.m4 b/config.m4
index 3559453..6eea7a5 100644
--- a/config.m4
+++ b/config.m4
@@ -220,7 +220,7 @@ if test "$PHP_APCU" != "no"; then
   LIBS_SAVED=$LIBS; LIBS=
   AC_SEARCH_LIBS([shm_open], [rt])
   LIBS=$LIBS_SAVED
-  AS_VAR_IF([ac_cv_search_shm_open], ["none required"], [],
+  AS_CASE([$ac_cv_search_shm_open], ["none required"|no], [],
     [PHP_ADD_LIBRARY([rt], [], [APCU_SHARED_LIBADD])])
 
   PHP_NEW_EXTENSION([apcu], [$apc_sources], [$ext_shared],, [$APCU_CFLAGS])