fix log2(Scalar::float_v) failure on MacOS

according to Lorenzo Moneta the Apple std::log2(float) implementation
only provides results with 2 ulp distance (like with MSVC)

refs #630

Signed-off-by: Matthias Kretz <kretz@kde.org>
fix Vc::Scalar atan/atan2 failures on MacOS

according to Lorenzo Moneta the Apple std::atan and std::atan2
implementations do not conform to the man page documentation provided on
Linux. The man pages say that some special input values return specific
results. But the std::atan/atan2 functions on MacOS apparently return
values that don't match exactly.

fixes #630

Signed-off-by: Matthias Kretz <kretz@kde.org>