Force FreeBSD ports to use python24

I use python 2.4 with trac, mod_python, but when i updated ports collection, it started to install new software with python2.5 as dependency.

By reading /usr/ports/Mk/bsd.python.mk , I found that I need to set PYTHON_DEFAULT_VERSION=python2.4 in /etc/make.conf and new software will be installed with python 2.4

After that new ports will use your existing python2.4.