fix wrong cmake var reference (PR #1164, by @dg0yt)

Author: daanx

CMakeLists.txt

@@ -195,7 +195,7 @@ if(MI_OVERRIDE)
       message(STATUS "  Use interpose to override malloc (MI_OSX_INTERPOSE=ON)")
       list(APPEND mi_defines MI_OSX_INTERPOSE=1)
       if (NOT MI_OSX_ZONE)
-        message(STATUS "  WARNING: interpose usually also needs zone overriding (use -DMI_OSX_INTERPOSE=ON)")
+        message(STATUS "  WARNING: interpose usually also needs zone overriding (use -DMI_OSX_ZONE=ON)")
       endif()
     endif()
     if(MI_USE_CXX AND MI_OSX_INTERPOSE)