Turn mempool_enabled() into a weak symbol

Before we had the following scheme:
mempool_enabled() would check mempool_use_allowed, and
libsystemd-shared would be linked with a .c file that provides mempool_use_allowed=true,
while other things would linked with a different .c file with mempool_use_allowed=false.

In the new scheme, mempool_enabled() itself is a weak symbol. If it's
not found, we assume false. So it only needs to be provided for libsystemd-shared,
where it can return false or true.

test-set-disable-mempool is libshared, so it gets the symbol. But then we
actually disable the mempool via envvar. mempool_enable() is called to check
its return value directly.

src/basic/hashmap.c

@@ -771,16 +771,15 @@ static void shared_hash_key_initialize(void) {
 static struct HashmapBase* hashmap_base_new(const struct hash_ops *hash_ops, enum HashmapType type  HASHMAP_DEBUG_PARAMS) {
         HashmapBase *h;
         const struct hashmap_type_info *hi = &hashmap_type_info[type];
-        bool up;
 
-        up = mempool_enabled();
+        bool use_pool = mempool_enabled && mempool_enabled();
 
-        h = up ? mempool_alloc0_tile(hi->mempool) : malloc0(hi->head_size);
+        h = use_pool ? mempool_alloc0_tile(hi->mempool) : malloc0(hi->head_size);
         if (!h)
                 return NULL;
 
         h->type = type;
-        h->from_pool = up;
+        h->from_pool = use_pool;
         h->hash_ops = hash_ops ?: &trivial_hash_ops;
 
         if (type == HASHMAP_TYPE_ORDERED) {