Fix Makefile and add VDE switch interface icon

Previous commit changed standard to C99, but the network part relies on
features from BSD/System V. So we're back to GNU99 for the moment.
=> Goal is to learn how system drivers on Linux work, to see whether I
   can find a pure Linux/C99 solution to extract information through
   ioctl() calls.

Add VDE switch interface icon: if the interface name is 'vde0', the
appropriate icon is set. This is for when I'm playing router and
masquerading packets for VMs, and using the VDE switch for that.

The battery now only displays time and power consumption/recharging
information if the amount of energy leaving/entering the battery is not
0 watt. This is to avoid division by 0 and weird numbers in the bar.