astro/boinc-setiathome: Fix build on powerpc64.

Code for AltiVec is written strictly with Mac OS X in mind exclusively and doesn't build on other systems.

Disable AltiVec on systems other than Mac OS X, which probably makes this run much slower, but at least it builds.

Tested on powerpc64 and amd64.

Hardware sponsored by IntegriCloud.

PR: 234145
Submitted by: pkubaj@anongoth.pl