fixed constant errors for gcc in c99 mode

C standard does not consider a `static const int` as a constant.
This is a problem for initializer, and ZSTD_STATIC_ASSERT().
Replaced by macro values

tests/fuzzer.c

@@ -920,7 +920,7 @@ static int basicUnitTests(U32 seed, double compressibility)
 
     /* custom formats tests */
     {   ZSTD_CCtx* const cctx = ZSTD_createCCtx();
-        static const size_t inputSize = CNBuffSize / 2;   /* won't cause pb with small dict size */
+        size_t const inputSize = CNBuffSize / 2;   /* won't cause pb with small dict size */
 
         /* basic block compression */
         DISPLAYLEVEL(4, "test%3i : magic-less format test : ", testNb++);