Provide a fixed version of atan2() under Windows, to workaround issues with infinities.

(will hopefully fix the cmath tests under Windows)
Try to fix test_log_base under Windows by lowering precision a bit
Merge branch 'master' of github.com:numba/numba into atan2_windows
Merge pull request #796 from pitrou/atan2_windows

Provide a fixed version of atan2() under Windows, to workaround issues w...