Make python2 usage more robust

After configuring Qt there will be an error message if a suitable python
version could not be found.

Add python2 configure test that
  - first looks for python2 in PATH
  - then looks for python in PATH
  - checks the Python version
  - stores the result in QMAKE_PYTHON2

Use $$QMAKE_PYTHON2 everywhere where we call python.

Pass $$QMAKE_PYTHON2 to gn for its exec_script feature.

Task-number: QTBUG-60164
Change-Id: I33de1273cbd20a787b3c8889d35280784dbcd5ae
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>