solves warnings when compiling with global XXH_STATIC_LINKING_ONLY

XXH_STATIC_LINKING_ONLY protection macro is intended to be triggered just before the include.
The main idea is to keep this setting local :
user module shall explicitly understand and accept the static linking restriction
which becomes transparent when triggering the macro at project level.
Global definition also triggers redefinition warnings for user modules which do locally define the macro.

This new version compiles lib and cli without warning when the macro is set globally.
That's not a scenario to be recommended, since it trades a local effect for a global one,
but it was easy enough to provide from zstd side.

lib/common/xxhash.c

@@ -104,7 +104,9 @@ static void  XXH_free  (void* p)  { free(p); }
 #include <string.h>
 static void* XXH_memcpy(void* dest, const void* src, size_t size) { return memcpy(dest,src,size); }
 
-#define XXH_STATIC_LINKING_ONLY
+#ifndef XXH_STATIC_LINKING_ONLY
+#  define XXH_STATIC_LINKING_ONLY
+#endif
 #include "xxhash.h"
 
 

lib/common/zstd_internal.h

@@ -49,6 +49,10 @@
 #include "error_private.h"
 #define ZSTD_STATIC_LINKING_ONLY
 #include "zstd.h"
+#ifndef XXH_STATIC_LINKING_ONLY
+#  define XXH_STATIC_LINKING_ONLY   /* XXH64_state_t */
+#endif
+#include "xxhash.h"               /* XXH_reset, update, digest */
 
 
 /*-*************************************

lib/compress/zstd_compress.c

@@ -13,8 +13,6 @@
 ***************************************/
 #include <string.h>         /* memset */
 #include "mem.h"
-#define XXH_STATIC_LINKING_ONLY   /* XXH64_state_t */
-#include "xxhash.h"               /* XXH_reset, update, digest */
 #define FSE_STATIC_LINKING_ONLY   /* FSE_encodeSymbol */
 #include "fse.h"
 #define HUF_STATIC_LINKING_ONLY

lib/compress/zstdmt_compress.c

@@ -25,8 +25,6 @@
 #include "threading.h"  /* mutex */
 #include "zstd_internal.h"   /* MIN, ERROR, ZSTD_*, ZSTD_highbit32 */
 #include "zstdmt_compress.h"
-#define XXH_STATIC_LINKING_ONLY   /* XXH64_state_t */
-#include "xxhash.h"
 
 
 /* ======   Debug   ====== */

lib/decompress/zstd_decompress.c

@@ -43,8 +43,6 @@
 *********************************************************/
 #include <string.h>      /* memcpy, memmove, memset */
 #include "mem.h"         /* low level memory routines */
-#define XXH_STATIC_LINKING_ONLY   /* XXH64_state_t */
-#include "xxhash.h"      /* XXH64_* */
 #define FSE_STATIC_LINKING_ONLY
 #include "fse.h"
 #define HUF_STATIC_LINKING_ONLY

lib/legacy/zstd_v07.c

@@ -13,12 +13,14 @@
 #include <string.h>     /* memcpy */
 #include <stdlib.h>     /* malloc, free, qsort */
 
-#define XXH_STATIC_LINKING_ONLY   /* XXH64_state_t */
-#include "xxhash.h"      /* XXH64_* */
+#ifndef XXH_STATIC_LINKING_ONLY
+#  define XXH_STATIC_LINKING_ONLY    /* XXH64_state_t */
+#endif
+#include "xxhash.h"                  /* XXH64_* */
 #include "zstd_v07.h"
 
-#define FSEv07_STATIC_LINKING_ONLY  /* FSEv07_MIN_TABLELOG */
-#define HUFv07_STATIC_LINKING_ONLY  /* HUFv07_TABLELOG_ABSOLUTEMAX */
+#define FSEv07_STATIC_LINKING_ONLY   /* FSEv07_MIN_TABLELOG */
+#define HUFv07_STATIC_LINKING_ONLY   /* HUFv07_TABLELOG_ABSOLUTEMAX */
 #define ZSTDv07_STATIC_LINKING_ONLY
 
 #include "error_private.h"
@@ -4536,7 +4538,8 @@ size_t ZBUFFv07_decompressContinue(ZBUFFv07_DCtx* zbd,
                     if (!decodedSize && !isSkipFrame) { zbd->stage = ZBUFFds_read; break; }   /* this was just a header */
                     zbd->outEnd = zbd->outStart +  decodedSize;
                     zbd->stage = ZBUFFds_flush;
-                    // break; /* ZBUFFds_flush follows */
+                    /* break; */
+                    /* pass-through */
             }   }
 
         case ZBUFFds_flush: