CMake build should specify C++ version

According to the Meson build, [this project requires at least C++11](https://github.com/imageworks/pystring/blob/master/meson.build#L8). This isn't specified in the CMake build, which means an older version of C++ might be used. We've seen this happen in practice.

```cmake
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_STANDARD 11)
```