Synchronize FindPythonLibs with upstream CMake
I was trying to make a debug build of ParaView on Windows (using CMake's Ninja generator, not sure if it matters). But the python modules were failing to link because the couldn't find python36_d.lib. It turns out our old FindPythonLibs.cmake in VTK was the culprit. Deleting it and letting the one from CMake 3.8.0-rc2 find the libraries resulted in a build that could link (but since it wasn't a clean configure I'm not sure what else might have broken, the variables from the old one were still there). We should update to use the one from upstream CMake.
@dgobbi I would have made an MR, but it is a non-trivial diff and I wasn't sure what I'd be breaking (and don't really have time at the moment to fix it). It looks like the updated one removes some variables that the one in VTK sets. If we update, we do need to update to the one from CMake 3.8 though since there was a bug in FindPythonLibs that is fixed in that release (cmake/cmake#16729 (closed)). It would be nice to do this for VTK 8.0, but I realize that is very soon.