--- a/cephes.asd
+++ b/cephes.asd
@@ -16,7 +16,7 @@
   (defclass makefile (source-file) ((type :initform "m")))
   (defmethod perform ((o load-op) (c makefile)) t)
   (defmethod perform ((o compile-op) (c makefile))
-    (let* ((lib-dir (system-relative-pathname "cephes" "scipy-cephes/"))
+    (let* ((lib-dir #P"@out@/scipy-cephes/")
            (lib (make-pathname :directory (pathname-directory lib-dir)
                                :name #+(or (and unix (not darwin)) windows win32) "libmd"
 			       #+(and darwin arm64) "libmd-arm64"
@@ -30,7 +30,7 @@
 	  (format *error-output* "Library ~S exists, skipping build" lib)
 	  (format *error-output* "Building ~S~%" lib))
       (unless built
-	(chdir (native-namestring lib-dir))
+	(chdir "scipy-cephes")
 	(run-program "make" :output t)))))
 
 (defsystem "cephes"