[core] use system xxhash lib if available

src/algo_xxhash.c

@@ -1,3 +1,8 @@
+#include "first.h"
+typedef int innocuous_typedef_to_quiet_empty_translation_unit_compiler_warning;
+
+#ifndef HAVE_XXHASH_H
+
 /*
  * xxHash - Extremely Fast Hash algorithm
  * Copyright (C) 2012-2020 Yann Collet
@@ -41,3 +46,5 @@
 #define XXH_IMPLEMENTATION   /* access definitions */
 
 #include "algo_xxhash.h"
+
+#endif /* ! HAVE_XXHASH_H */

src/algo_xxhash.h

@@ -1,8 +1,19 @@
+#ifndef INCLUDED_ALGO_XXHASH_H
+#define INCLUDED_ALGO_XXHASH_H
+#include "first.h"
+
 /*(lighttpd customization)*/
 /*#define XXH_NO_INLINE_HINTS 1*/
 /*#define XXH_REROLL 1*/
 #define XXH_NO_LONG_LONG
 
+
+#ifdef HAVE_XXHASH_H
+
+#include <xxhash.h>
+
+#else  /* ! HAVE_XXHASH_H */
+
 /*
  * xxHash - Extremely Fast Hash algorithm
  * Header File
@@ -4769,3 +4780,7 @@ XXH128_hashFromCanonical(const XXH128_canonical_t* src)
 #if defined (__cplusplus)
 }
 #endif
+
+#endif /* ! HAVE_XXHASH_H */
+
+#endif