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

[Mark Dickinson]

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

One other thought:  we should check that it's not pow that's at fault here, rather than log2.  The test uses math.log2(2.0**n).  It would probably be better off using math.log2(ldexp(1.0, n)), or similar:  the libm pow operation is also notorious for inaccuracies (due to poor implementations or otherwise) on various platforms.

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