...along with PYTHON, PYTHON_CFLAGS, and PYTHON_LIBS.  (PYTHON_VERSION is
apparently set by AM_PATH_PYTHON.  When explicitly passing in PYTHON_CFLAGS and
PYTHON_LIBS under --enable-python=system, AM_PATH_PYTHON does not get called,
but PYTHON_VERSION needs to be set, so needs to be passed in explicitly.)

Change-Id: Ia8c3877fffd8dcc179a56be04627c61a667d8cb0
Reviewed-on: https://gerrit.libreoffice.org/73662
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>