In commit a6fee09484 (file: Add CONFIGURE subcommand, 2020-03-06,
v3.18.0-rc1~584^2) we accidentally treated relative path outputs
with respect to the current working directory. Treat them with
respect to the current binary directory instead.
Fixes: #20885
Extend the `file()` command with a new `CONFIGURE` subcommand that
behaves the same as `string(CONFIGURE)` except that it writes the
resulting output immediately to a file.
Fixes: #20388
