The previous check used `env::var(...).is_ok()`, which only tested for
the variable's presence and ignored its value. Setting the variable to
`0` or `false` counter-intuitively still disabled gtk-layer-shell.
Introduce `env_flag_enabled` helper that treats `0`, `false`, `no`,
`off`, and the empty string as falsy (case-insensitive).
11311bee0e