We don't require any build steps other than the standard meson
commands, but entering a toolbox and running 2-3 meson commands
still adds some friction.
Add a small convencience script that
- finds the toplevel source directory (similar to `jhbuild make`)
- enters the speficied toolbox (or the configured default)
- automatically picks a build directory based on the toolbox name
- builds and installs the project to /usr (inside the toolbox)
It also allows specifying meson -D options to change the build
configuration, and to optionally run `meson dist`.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2935>
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