glx: turn LIBGL_ALLOW_SOFTWARE into a boolean

Instead of setting based on set/unset, allow users to use boolean values.
In the help string, use `ALLOW=true` instead of `ALLOW=1` as it's clearer IMO.

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
1 file changed