Wrap subrandr build process into meson custom target.
Meson's custom target allow only file output (without path component),
so additionall copy step is added to get the library. Dummy stamp file
is used to connect custom target to dependency, the file is actually not
created, so target will be called each time, but it's fine, because
cargo will resolve it's own dependencies this way and build if needed.
This is glued only for static lib, this is what we need for our CI
script.
Kacper Michajłow
e75a174743