master does not compile with clang++

With clang++ 9.0.0:

```
$ CXX=clang++ make
[...]
../../dsp/v4f_IIR2.h:51:11: error: no matching function for call to 'v4f_map'
                                cos = v4f_map<__builtin_cosf> (w);
```

All errors and warnings should be fixed. I can do it myself, but you don't accept PRs, do you?