Xonotic does not build out of the box for POWER, because the Makefile uses the
-march compiler flag, which does not exist on PowerPC. It can be fixed by replacing this line in the Makefile:
CFLAGS ?= -pipe -march=native -mtune=native -flto=auto
...with this line:
CFLAGS ?= -pipe -mcpu=native -mtune=native -flto=auto
After making this change, the build instructions should work.