[lib] Make lib compatible with -Wfall-through excepting legacy

Switch to a macro `ZSTD_FALLTHROUGH;` instead of a comment. On supported
compilers this uses an attribute, otherwise it becomes a comment.

This is necessary to be compatible with clang's `-Wfall-through`, and
gcc's `-Wfall-through=2` which don't support comments. Without this the
linux build emits a bunch of warnings.

lib/compress/zstd_compress_internal.h

@@ -486,7 +486,7 @@ MEM_STATIC int ZSTD_disableLiteralsCompression(const ZSTD_CCtx_params* cctxParam
         return 1;
     default:
         assert(0 /* impossible: pre-validated */);
-        /* fall-through */
+        ZSTD_FALLTHROUGH;
     case ZSTD_lcm_auto:
         return (cctxParams->cParams.strategy == ZSTD_fast) && (cctxParams->cParams.targetLength > 0);
     }