BUILD: Makefile: disable shared cache on AIX 5.1

BUILD: Makefile: disable shared cache on AIX 5.1

AIX 5.1 is missing the following builtins used for atomic locking of the
shared inter-process cache :

   .__sync_val_compare_and_swap_4
   .__sync_lock_test_and_set_4
   .__sync_sub_and_fetch_4

Let's simply use the private cache by default since nobody cares on
such old systems. No test was made on a more recent version.