Increase x-compatibility

lib/zstd.h

@@ -1802,7 +1802,15 @@ static
 #ifdef __GNUC__
 __attribute__((__unused__))
 #endif
+	
+#if defined(__clang__) && __clang_major__ >= 5
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wzero-as-null-pointer-constant"
+#endif
 ZSTD_customMem const ZSTD_defaultCMem = { NULL, NULL, NULL };  /**< this constant defers to stdlib's functions */
+#if defined(__clang__) && __clang_major__ >= 5
+#pragma clang diagnostic pop
+#endif
 
 ZSTDLIB_STATIC_API ZSTD_CCtx*    ZSTD_createCCtx_advanced(ZSTD_customMem customMem);
 ZSTDLIB_STATIC_API ZSTD_CStream* ZSTD_createCStream_advanced(ZSTD_customMem customMem);