C99 and older GCC fixes

contrib/declib/examples/simple.c

@@ -9,6 +9,7 @@
  */
  
 #include <stddef.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

contrib/declib/zstddeclib-in.c

@@ -45,8 +45,12 @@
  * 
  * Note: It's important that none of these affects 'zstd.h' (only the
  * implementation files we're amalgamating).
+ * 
+ * Note: MEM_MODULE stops xxhash redefining BYTE, U16, etc., which are also
+ * defined in mem.h (breaking C99 compatibility).
  */
 #define DEBUGLEVEL 0
+#define MEM_MODULE
 #define XXH_NAMESPACE ZSTD_
 #define XXH_PRIVATE_API
 #define XXH_INLINE_ALL