Finally, we generate osdef.h using the following function:

function(generate_osdef_h)
  find_program(BASH_EXECUTABLE bash)

  execute_process(
    COMMAND
      ${BASH_EXECUTABLE} osdef.sh ${CMAKE_CURRENT_BINARY_DIR}
    WORKING_DIRECTORY
      ${CMAKE_CURRENT_LIST_DIR}
    )
endfunction()

In order to generate osdef.h in ${CMAKE_CURRENT_BINARY_DIR}/src/auto instead of src/auto, we had to adapt osdef.sh to accept ${CMAKE_CURRENT_BINARY_DIR} as a command line argument.

Inside osdef.sh, we check whether this argument is given:

if [ $# -eq 0 ]
  then
    # there are no arguments
    # assume the target directory is current directory
    target_directory=$PWD
  else
    # target directory is provided as argument
    target_directory=$1
fi

And then, we generate ${target_directory}/auto/osdef.h. To do so, we also had to adjust the following compilation line inside osdef.sh:

$CC -I. -I$srcdir -I${target_directory} -E osdef0.c >osdef0.cc