[issue11888] Add C99's log2() function to the math library

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

> You know what? Mac OS X log2 is less accurate than Python log2!

That doesn't surprise me much.  Though it's probably still true that log2 from OS X is more accurate than our log2 for some other values.  It's just that getting the answer wrong for a power of 2 is a 'high-visibility' error.

Testing log2 sounds long-winded and error-prone.  I'd suggest just marking that test as an expected failure on Tiger.  BTW, I don't see any such failure on Snow Leopard.

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