Bug#949864: qgis: FTBFS with default python3.8

[Gianfranco Costamagna]

control: tags -1 important
(I don't remember how to tag it as experimental only, and I don't want autoremoval to start kicking!)

> We just need to add a test for python3 >= 3.8 and set PYTHON_LIBRARY
> accordingly.

yes, but I don't usually want to add more code to rules file, they are usually kept forever :)
But if you plan to backport, so its probably something useful to do.

BTW, I tried a plain cmake .. && make, and python was correctly detected...
Maybe its time to drop that check completely?

(sorry if I didn't understand correctly the reason for it!)

Gianfranco