CMAKE custom compiler not overwriting

Question

In a CMAKE project, I need to define a custom command for a file type (.osl) which is compiled by a tool (oslc) which compiles to another file type (.oso)

I managed to do it by a function that I can run on a list of source files:

find_program(OSLC_EXECUTABLE oslc)
function(compile_osl out_var)
    set(result)
    foreach(osl_f ${ARGN})
        file(RELATIVE_PATH osl_f_base ${CMAKE_CURRENT_SOURCE_DIR} ${osl_f})
        string(REGEX REPLACE "\\.osl$" ".oso" oso_f ${osl_f_base})
        set(oso_f "${CMAKE_CURRENT_BINARY_DIR}/${oso_f}")
        get_filename_component(oso_f_dir ${oso_f} DIRECTORY)
        file(MAKE_DIRECTORY ${oso_f_dir})
        add_custom_command(OUTPUT ${oso_f}
            COMMAND ${OSLC_EXECUTABLE} ${osl_f} -o ${oso_f}
            DEPENDS ${osl_f}
            WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
            COMMENT "Creating compiled OSL file ${oso_f}"
            VERBATIM
            )
        list(APPEND result ${oso_f})
    endforeach()
    set(${out_var} "${result}" PARENT_SCOPE)
endfunction()

Thanks to the DEPENDS directive, the compiler is only run if the sources are newer than the output files. However, the compiler does not overwrite output files, so it does not work.

Deleting with file(REMOVE ...) just before the custom command does not work, as it deletes all files, not only the ones requiring recompilation. Also, it deletes at cmake execution, not at make time.

I could maybe define another custom command with "rm" but this not cross platform (I would need to add specific lines for Windows, which I do not like).

Any idea?

Thanks!