Meson: use modern features and fix deprecations

[danirabbit]

Can't use install_symlink yet because of a bug in our version of Meson

[ryonakano]

Maybe we can replace the following join_paths too?

ryo@lb450m:~/work/tmp/default-settings (danirabbit/modern-meson =)$ git grep -n join_paths
meson.build:9:datadir = join_paths(prefix, get_option('datadir'))
meson.build:11:schemadir = join_paths(datadir, 'glib-2.0', 'schemas')
meson.build:15:symlink = join_paths(meson.current_source_dir (), 'meson', 'create-symlink.sh')
meson.build:20:    install_dir: join_paths(sysconfdir, 'profile.d'),
meson.build:27:    install_dir: join_paths(sysconfdir, 'sudoers.d')
meson.build:33:    install_dir: join_paths(datadir, 'cups', 'data')
meson.build:39:    install_dir: join_paths(sysconfdir, 'gtk-3.0')
meson.build:45:    install_dir: join_paths(datadir, 'applications')
overrides/meson.build:13:        install_dir: join_paths(schemadir)
overrides/meson.build:21:        install_dir: join_paths(schemadir)
skel/meson.build:1:skeldir = join_paths(sysconfdir, 'skel')
skel/meson.build:7:        install_dir: join_paths(skeldir, '.config')
skel/meson.build:15:        install_dir: join_paths(skeldir, '.local', 'share')
ryo@lb450m:~/work/tmp/default-settings (danirabbit/modern-meson =)$ 

[ryonakano]

Code looks good and ninja install works too.