diff --git a/m4/ax_pthread.m4 b/m4/ax_pthread.m4
index 4c4051ea3..aefb81af3 100644
--- a/m4/ax_pthread.m4
+++ b/m4/ax_pthread.m4
@@ -261,8 +261,8 @@ if test "x$ax_pthread_clang" = "xyes"; then
         # -pthread does define _REENTRANT, and while the Darwin headers
         # ignore this macro, third-party headers might not.)
 
-        PTHREAD_CFLAGS="-pthread"
-        PTHREAD_LIBS=
+        PTHREAD_CFLAGS=
+        PTHREAD_LIBS="-pthread"
 
         ax_pthread_ok=yes