Bug#832870: 0ad: FTBFS: configure: error: python was not found in $PATH

[peter green]

tags 832870 +patch
thanks

This is caused by a missing build-dependency on python. This was 
previously masked by python being pulled in indirectly.

While working on this issue I also found the clean target was broken. So 
I fixed that too.

I have uploaded this fix to raspbian, debdiff at 
https://debdiffs.raspbian.org/main/0/0ad/0ad_0.0.20-1+rpi1.debdiff