Default to G4 on darwin for 10.5+

[Bradley Lucier]

Re:

> This patch defaults ppc compiles to G4 for compiles that target  
> 10.5 or later.

Would it be a good idea, then, for Apple's automatic regression  
tester to start specifying

RUNTESTFLAGS="--target_board 'unix{-mcpu=G4}'"

as a make check option?  (I presume it's not running on a G3  
machine.) This would have caught all automatically vectorized tests  
that were recently broken.

Brad