[CMake] howto not remove an output in make clean

[Gaspard Bucher]

Hi list !

I have a command that generates a "foo.cpp" file from a ragel parser
definition "foo.rl". I am doing an out-of-source build but I would
like to keep the generated file in the source so that it is committed
and end users do not need to install ragel because "foo.cpp" is newer
then "foo.rl".

Is there a way to tell "make clean" to *not* remove the output from
the custom command "foo.cpp" ?

Gaspard