The idea is to write to a temp file which contains generator
expressions, and at generate time, evaluate the generator expressions,
and write the result to a file.
Because executables on Windows are limited in the length of command line
it is possible to use, it is common to write command line arguments to a
file instead and specify the file as a source of arguments.
This new FILE(GENERATE) subcommand allows the use of generator
expressions to create such files so that they can be used with
add_custom_command for example.
Stephen Kelly
Ben Boeckel