Respect CMake defaults for `CMAKE_CXX_FLAGS`

**Is your feature request related to a problem? Please describe.**

In #24456 we discovered that rules_cc has a fundamentally different C/C++ toolchain model than CMake: namely, that it ignores `CXX` and expects `CC` to be a compiler driver than can handle both C and C++. As such, we had CMake not even bother to search for C++ as a language via the `project()` command.

However, CMake has built-in defaults for [`CMAKE_<LANG>_FLAGS`](https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS.html), like `-O3 -DNDEBUG` under a release build. When `CXX` is not specified as a language for the project, these defaults for `CMAKE_CXX_FLAGS` are not initialized. In the context of #24526, this may be problematic; at least, when running with [`cmake --warn-uninitialized`](https://cmake.org/cmake/help/v4.3/manual/cmake.1.html#cmdoption-cmake-warn-uninitialized), something like the following will be produced:

```
CMake Warning (dev) at CMakeLists.txt:175 (separate_arguments):
  uninitialized variable 'CMAKE_CXX_FLAGS'
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at CMakeLists.txt:175 (separate_arguments):
  uninitialized variable 'CMAKE_CXX_FLAGS_RELEASE'
This warning is for project developers.  Use -Wno-dev to suppress it.
```

**Describe the solution you'd like**

Either respect CMake's default `CMAKE_CXX_FLAGS` by re-enabling it as a language in the `project` command, or at least guard the logic from 24526 behind whether those variables are non-empty. 

If the former, in order to support Bazel's compiler-driver-based model, the CML should probably enforce that `CMAKE_C_COMPILER` and `CMAKE_CXX_COMPILER` are the same.

**Describe alternatives you've considered**

The two alternative options I see at this point are listed above.

**Additional context**

N/A

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