[CMake] How to generate preprocessed file?

[Marcel Loose]

Hi all,

I was trying to figure out how to generate a preprocessed file. By
browsing the CMake modules and by printing all CMake variables I
discovered CMAKE_<LANG>_CREATE_PREPROCESSED_SOURCE variables. That
suggests that there is support for generating preprocessed files. 

However, there's no mention of these variables in the documentation, or
the FAQ or the Wiki. How should I tell CMake to add rules to my Unix
Makefiles for generating preprocessed (.i) files?

Best regards,
Marcel Loose.