Put this into /etc/apache2/other/jenkins.conf:
ProxyPass         /jenkins  http://localhost:8009/jenkins
ProxyPassReverse  /jenkins  http://localhost:8009/jenkins
ProxyRequests     Off
<Proxy http://localhost:8009/jenkins*>
    Order deny,allow
    Allow from 127.0.0.1
</Proxy>
Then execute these commands:
sudo defaults write /Library/Preferences/org.jenkins-ci httpPort 8009
sudo defaults write /Library/Preferences/org.jenkins-ci prefix /jenkins
sudo launchctl stop org.jenkins-ci