Install AltiVec libvpx and modify build system (G4, G5 only)

```
In issue 26 a user brought up poor WebM speed. This is undoubtedly due to issue 
7, but we should look into writing a PPC-specific decoder path along the lines 
of the ARM and x86 code that already exists to at least speed up the decoding 
step even if rendering still sucks.
```

Original issue reported on code.google.com by `classi...@floodgap.com` on 5 Feb 2011 at 5:22
- Blocking: #51, #51
