Xonotic does not build out of the box for POWER, because the Makefile uses the
-march compiler flag, which is a machine-dependent option that exists on x86 but 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.
Xonotic logs some messages like this:
Skeletal animation uses generic code path (SSE not compiled in) DPSOFTRAST not available (SSE2 not compiled in)
It would be interesting to try porting these SIMD code paths to POWER to see if performance improves.