build: check for unw_set_cache_size()

This may not be available on older libunwind versions such as 1.2.

src/libsysprof/preload/sysprof-memory-collector.c

@@ -64,7 +64,9 @@ collector_init_ctor (void)
 {
 #ifdef ENABLE_LIBUNWIND
   unw_set_caching_policy (unw_local_addr_space, UNW_CACHE_PER_THREAD);
+# ifdef HAVE_UNW_SET_CACHE_SIZE
   unw_set_cache_size (unw_local_addr_space, 1024, 0);
+#endif
 #endif
   sysprof_collector_init ();
   collector_ready = TRUE;