Choosing interfaces under /sys/class/backlight, UPOWER_ENABLE_DEPRECATED defines.

Now the user is able to define which interface under /sys/class/backlight will be used to manage the brightness.
Despite I'm investigating a "better" method using XRandr, I have no guarantees about if it can be done or not, so I post the pull request with this method.

`UPOWER_ENABLE_DEPRECATED` has also been added, to enable compilation on systems which use newer versions of upower.