Another attempt to fix CMake Python detection on USE_PYTHON3 status changes

Dave Moxeyrequested to merge
dmoxey/nektar:fix/python-cmake into master

This MR fixes another issue in the USE_PYTHON3 detection logic, since the PYTHON_LIBRARIES variable inside FindPythonLibsNew is not cached and is unset on multiple configurations. This currently prevents builds on macOS using Python, since neither MacPorts nor Homebrew explicitly link libboost_python against the Python libraries.

Edited by Dave Moxey

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