Scope out usage of CUDA as cmake language

Idea: do not rely on nvcc_wrapper for the cuda-vs-host compiler magic, but use native CMake stuff (assuming cmake version is 3.18 or higher). Possible approach: set a var `CXX_LANG` which evaluates to `CXX` for non-cuda builds, and to `CUDA` for cuda builds. Then manually set the language of all src files to `${CXX_LANG}`, to force compilation with the proper compiler.

I speculate that this would also allow to get stuff like
```
target_compile_features(mylib PRIVATE cxx_constexpr)
```
to work correctly. I _think_ the nvcc_wrapper layer is somehow blocking communication between the compiler and cmake, preventing correct detection of cxx features. By using native CMake support for CUDA, I suspect this could be fixed, since cmake would invoke nvcc for compiling CUDA sources, folllowed by an invoke of the host compiler on the generated intermediate cpp source.