b2 tries to open .rsp files

If you have a custom `user-config.jam` containing e.g.

```
using msvc : : "C:/Program Files (x86)/Microsoft Visual Studio/2017/Professional/VC/Tools/MSVC/14.16.27023/bin/Hostx64/x64/cl.exe" ;
```

And you try to build Boost with

```
bootstrap
b2 --user-config=C:/path/to/user-config.jam
```

Then it works fine with Boost 1.69.0, but with 1.70.0 you get a load of dialog boxes saying windows doesn't know how to open .rsp files.