Fix an unused variable warning for {snappy,lz4}-free compilation

```
upstream/c-blosc/blosc/blosc.c:1698:8: warning: unused variable 'sbuffer'
      [-Wunused-variable]
  char sbuffer[256];
       ^
```

Fixed with:

```c
#if (defined(HAVE_LZ4) && defined(LZ4_VERSION_MAJOR)) || (defined(HAVE_SNAPPY) && defined(SNAPPY_VERSION))
```

Other possible approaches include:

- `char sbuffer[256] __attribute__ ((unused));` (works with GCC and clang)

- `sbuffer[0] = 0; // to avoid an unused variable warning`

- refactoring to have `*version = strdup(...)` inside each case's block