Use sbcl in the absence of ccl on OS X

modules/prelude-common-lisp.el

@@ -54,7 +54,8 @@
         (sbcl ("sbcl" "--noinform") :coding-system utf-8-unix)))
 
 ;; select the default value from slime-lisp-implementations
-(if (eq system-type 'darwin)
+(if (and (eq system-type 'darwin)
+         (executable-find "ccl"))
     ;; default to Clozure CL on OS X
     (setq slime-default-lisp 'ccl)
   ;; default to SBCL on Linux and Windows