BUILD: hpack: make sure the hpack table can still be built standalone

Recent commit 2bdcc70fa7 ("MEDIUM: hpack: use a pool for the hpack table")
made the hpack code finally use a pool with very unintrusive code that was
assumed to be trivial enough to adjust if the code needed to be reused
outside of haproxy. Unfortunately the code in contrib/hpack already uses
it and broke the oss-fuzz tests as it doesn't build anymore.

This patch adds an HPACK_STANDALONE macro to decide if we should use the
pools or malloc+free. The resulting macros are called hpack_alloc() and
hpack_free() respectively, and the size must be passed into the pool
itself.
diff --git a/include/common/hpack-tbl.h b/include/common/hpack-tbl.h
index 44c0cc1..6d529f2 100644
--- a/include/common/hpack-tbl.h
+++ b/include/common/hpack-tbl.h
@@ -136,6 +136,17 @@
 extern const struct http_hdr hpack_sht[HPACK_SHT_SIZE];
 extern struct pool_head *pool_head_hpack_tbl;
 
+/* when built outside of haproxy, HPACK_STANDALONE must be defined, and
+ * pool_head_hpack_tbl->size must be set to the DHT size.
+ */
+#ifndef HPACK_STANDALONE
+#define hpack_alloc(pool)      pool_alloc(pool)
+#define hpack_free(pool, ptr)  pool_free(pool, ptr)
+#else
+#define hpack_alloc(pool)      malloc(pool->size)
+#define hpack_free(pool, ptr)  free(ptr)
+#endif
+
 extern int __hpack_dht_make_room(struct hpack_dht *dht, unsigned int needed);
 extern int hpack_dht_insert(struct hpack_dht *dht, struct ist name, struct ist value);
 
@@ -243,7 +254,7 @@
 	if (unlikely(!pool_head_hpack_tbl))
 		return NULL;
 
-	dht = pool_alloc(pool_head_hpack_tbl);
+	dht = hpack_alloc(pool_head_hpack_tbl);
 	if (dht)
 		hpack_dht_init(dht, pool_head_hpack_tbl->size);
 	return dht;
@@ -252,7 +263,7 @@
 /* free a dynamic headers table */
 static inline void hpack_dht_free(struct hpack_dht *dht)
 {
-	pool_free(pool_head_hpack_tbl, dht);
+	hpack_free(pool_head_hpack_tbl, dht);
 }
 
 #endif /* _COMMON_HPACK_TBL_H */