GitLab

Computing the opposite half-handle of a half-handle with index -1,
we obtain a handle with idx=-2, which should also be treated as invalid.
Thanks to Heng Liu for the observation.

cppcheck currently returns a bogus error:
```
OpenVolumeMesh/OpenVolumeMesh/src/OpenVolumeMesh/Core/Handles.hh:71:44: error: syntax error [syntaxError]
bool is_valid() const { return idx_ != -1; }
```

CMake/windows:do not force static build, follow CMake defaults. emscripten should not need special treatment either (untested).