don't try to use Python 3 (if present) in configure

In Arch Linux, the binary /usr/bin/python is Python 3, Python 2.7 is installed in /usr/bin/python2 (and /usr/bin/python2.7). When running ./configure with Python 3 installed, it fails since it tries to use /usr/bin/python. Setting PYTHON_VERSION as recommended does not help. The only way of building it is to uninstall the Python 3 package.

I suggest to respect PYTHON_VERSION=2.7 as advertised in ./configure --help, such that you can force configure to use the python2.7 binary.