diagnostic: -Wunused-but-set-variable is a problem until GCC 6.4
diagnostic: -Wunused-but-set-variable is a problem until GCC 6.4

Previously this had been treated as fixed in 6.0 (in #277), but in
reality it was not fixed until 6.4.

Tested on godbolt with the following snippet:

```
int test() {
    struct simde__m128i_private {
        int  i32f __attribute__((__vector_size__(16)));
    };
  simde__m128i_private r_, a_;
  r_.i32f = ~(a_.i32f);
  return r_.i32f[0];
}
```

Fixes #453.

nemequ/simde,nemequ/simde,nemequ/simde