Replace M_LOG2E and M_LN2 with custom macros.

For these to exist we would need to define `_USE_MATH_DEFINES` before
`cmath` or `math.h` is first included.  However, we don't
control the include order for projects outside Eigen, so even defining
the macro in `Eigen/Core` does not fix the issue for projects that
end up including `<cmath>` before Eigen does (explicitly or transitively).

To fix this, we define `EIGEN_LOG2E` and `EIGEN_LN2` ourselves.

test/packetmath.cpp

@@ -495,7 +495,7 @@ void packetmath() {
 // c++11 has std::log2 for real, but not for complex types.
 template <typename Scalar>
 Scalar log2(Scalar x) {
-  return Scalar(M_LOG2E) * std::log(x);
+  return Scalar(EIGEN_LOG2E) * std::log(x);
 }
 
 template <typename Scalar, typename Packet>