[CMake] Redirect output from add_custom_target()

[Robert Dailey]

I'd like to redirect stderr to stdout in the command invoked by
add_custom_target(). Here is my usage:

        add_custom_target( ${doc_target_name}
            DEPENDS ${generated_conf}
            COMMAND ${doxygen_exe} ${generated_conf}
            WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
            COMMENT "Generating documentation using Doxygen"
        )

Is there any way I can do this?