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.
diff --git a/Makefile b/Makefile
index 9bb24f1..297d97c 100644
--- a/Makefile
+++ b/Makefile
@@ -381,7 +381,7 @@
 # AIX 5.1 only
 ifeq ($(TARGET),aix51)
   set_target_defaults = $(call default_opts, \
-    USE_POLL USE_LIBCRYPT USE_OBSOLETE_LINKER)
+    USE_POLL USE_LIBCRYPT USE_OBSOLETE_LINKER USE_PRIVATE_CACHE)
   TARGET_CFLAGS   = -Dss_family=__ss_family -Dip6_hdr=ip6hdr -DSTEVENS_API -D_LINUX_SOURCE_COMPAT -Dunsetenv=my_unsetenv
   DEBUG_CFLAGS    =
 endif