Having a template file with replacement macros is a bit cleaner than
encoding the setup of the file in `meson.build` and piping that through
`echo` with caputred variables.
Unfortunately there appears to be no support for repetition, to e.g.
expand a list of platform names into the various uses (array with comma-
separation, or individual `extern struct` declarations, forcing us to
have some form of string expansion within `meson.build`).
