Really update libarcus to 4.8

N.B. the wrinkle in cmake build here.
find_package(Pythoninterp 3.4 REQUIRED) can find your install Python 3.9
instead of the DEFAULT Python. Force the correct one with a few patches.

PR: ports/251236
Submitted by: sko@rostwald.de
Reported by: trombik1973@gmail.com