Based on user requests, I have created a Java5 branch. To get it, clone the git repo and get the branch:
# git clone git://simplecaptcha.git.sourceforge.net/gitroot/simplecaptcha/simplecaptcha
# cd simplecaptcha
# git checkout java1.5
If you get errors during the build about various things related to javax.servlet, set the $CATALINA_HOME environment variable to a directory containing the servlet-api.jar. For me, this is:
# export CATALINA_HOME=/usr/apache-tomcat-6.0.29
I have done my best, but do not have a 1.5 SDK on my system currently, and there does not appear to be one that is easily available for OS X. Because of this I have not provided a downloadable jar. If someone is able to build it against an actual Java5 JDK and jar it up, I will be happy to add it to the mix.