use enable_language as needed in cmake outputs?

As an aside, it would also be useful to check inside the `SDL2Config.cmake` config file whether the `CXX` language is actually enabled.
Because if it is not, the build process will fail at a later time.
See e.g. [this issue](https://issuetracker.google.com/issues/236447801) I created earlier this year.

I think the cmake config file might need something like:
```cmake
include(CheckLanguage)
check_language(CXX)
if(NOT CMAKE_CXX_COMPILER)
message(WARNING "SDL2 requires CXX, but it not enabled.")
endif()
```
[`CheckLanguage`](https://cmake.org/cmake/help/latest/module/CheckLanguage.html) documentation

Or skip the warning and do `enable_language(CXX)` yourself.

_Originally posted by @madebr in https://github.com/google/prefab/issues/161#issuecomment-1275363135_