FIX #3566: redefine isfinite alias in sklearn

npy_isfinite from npy_math.h depends on the compiler used to build
numpy. If we build scikit-learn with a different compiler (e.g. MSVC
vs MinGW) this can cause link errors.

Therefore we redefine the isfinite macro in a scikit-learn specific header
file.
Merge pull request #3611 from ogrisel/fix-isfinite-windows

[MRG+1] FIX #3566: redefine isfinite alias in sklearn