COMP: Pass CMAKE_CXX_FLAGS for each build type only if needed
COMP: Pass CMAKE_CXX_FLAGS for each build type only if needed

If using a multi-config generator, all type are passed. If you using
a single build type generator, only flag associated with the current
build type are passed.

Additionally, since all external projects are C++, the C_FLAGS is not
passed. This prevents the following CMake warnings from being reported:

```
CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_CXX_FLAGS_RELEASE
    CMAKE_C_FLAGS_DEBUG
    CMAKE_C_FLAGS_RELEASE
```

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