configure.ac: Exit early if PYTHON is not Python 2

JHBuild still only supports Python 2. However, automake doesn't seem to provide a way to choose between python 2 and 3, and it just accepts the default one if its version number is high enough. Therefore, on systems using Python 3 by default, JHBuild will be configured to use Python 3, causing JHBuild to crash immediately because of invalid syntax. In order to avoid installing a broken version of JHBuild, we should check the version ourselves and exit early if PYTHON is not Python 2.