Add some docs for file(GENERATE).
file: Add GENERATE command to produce files at generate time

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.
Merge topic 'genex-generate-file' into next

8e15f1a Add some docs for file(GENERATE).
Merge topic 'genex-generate-file' into next

b983a58 file: Add GENERATE command to produce files at generate time