BUG: improve log2 windows compiler fallback of log2

Fallback (similar to python3.4 math.log2) provides int(log(2**i)) == i
in default rounding mode.
Non-default rounding modes not tested but if it does not work we are at
least not worse than python.
Closes gh-4787
Merge pull request #4790 from juliantaylor/log2-windows

BUG: improve log2 windows compiler fallback of log2