[CMake] Two toolchains simultaneously within the same project?

[Ingolf Steinbach]

Hi.

2012/9/28 David Cole <david.cole at kitware.com>:
> Please try with a more recent CMake version. The most recent release is
> 2.8.9. ExternalProject was first introduced in 2.8.0, and many many issues
> have been fixed in it over the last 2 years.

I am now using CMake 2.8.10.2, and indeed, ExternalProject now allows
me to use a different toolchain in the external project. Thanks a lot,
David, for the hint.

However, during generation phase, cmake emits the following messages:

-- Configuring done
-- Generating done
CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_TOOLCHAIN_FILE

As this is exactly what was intended (not passing the toolchain
specification to the external project), I was expecting to be able to
avoid this warning by including
    CMAKE_ARGS -UCMAKE_TOOLCHAIN_FILE
in the ExternalProject_Add(...). Unfortunately, the warning persists.
How can I avoid this warning?

Kind regards
Ingolf