[libclc] Fix target dependency

The prepare target was depending on the output of a custom command, but
wasn't the full path to that file. This tripped up CMake if the file was
removed as it didn't know how to rebuild that file.

Author: frasercrmck

libclc/cmake/modules/AddLibclc.cmake

@@ -393,7 +393,7 @@ function(add_libclc_builtin_set)
   endif()
 
   # Add a 'prepare' target
-  add_custom_target( prepare-${obj_suffix} ALL DEPENDS ${obj_suffix} )
+  add_custom_target( prepare-${obj_suffix} ALL DEPENDS ${libclc_builtins_lib} )
   set_target_properties( "prepare-${obj_suffix}" PROPERTIES FOLDER "libclc/Device IR/Prepare" )
 
   # Also add a 'prepare' target for the triple. Since a triple may have