You can type sequence of commands in crontab, first changedir to ezpublish_root, then execute the script e.g:
# This must be set to the directory where eZ publish is installed.
EZPUBLISHROOT=/home/noamse/public_html/site
# Location of the PHP Command Line Interface binary.
PHP=/usr/local/bin/php
# Executes the runcronjobs.php script every 15th minute.
0,15,30,45 * * * * cd $EZPUBLISHROOT; $PHP -C runcronjobs.php -q 2>&1
your suggestion seems to be working. i have tried to use it on the new newsletter extention, so for others interested, i am posting here the crontab line i have used: