[CMake] Set Different Project Languages on Different Platforms?

[digitalriptide]

Is it possible to set different project languages on different
platforms? I was thinking of simply detecting the platform and having
different project calls for different platforms, but variables such as
${CMAKE_SYSTEM_NAME} don't seem to be set until *after* project is
called.

The reason I ask is that, on OS X, qt4 does not seem to require a CC
definition, but on Linux it does.

Thanks for input, cheers!