CMake: set_languages("c++17"), language not set in CMakeLists.txt

Create an xmake project with:
```
set_languages("c++17")
```

Create a CMakeLists.txt project file with xmake:
```
xmake project -k cmake
```

The resulting CMakeLists.txt does not have the language standard set for the target, eg:

```
target_compile_features(my_target PRIVATE cxx_std_17)
```

