The gradle.build build file can be used to recreate the Sage.jar on Windows, Linux or Mac.
After cloning the repository, you can re-create the Sage.jar by running the sageJar task
Linux Users
./gradlew sageJar
Windows Users
gradlew.bat sageJar
Keep in mind the first time you run gradlew it will take some time has it has to download some dependencies.
After cloning the repository, you can create the Eclipse Project Files by running the commands cleanEclipse eclipse
./gradlew cleanEclipse eclipse
You can open the project in eclipse and it will be fully configured as a Java Project.
You cannot recreate the Sage.jar from within eclipse, but it will compile files, etc. When you need to rebuild the Sage.jar simple use the sageJar task.
You can find information on how to install the Gradle Plugin here http://www.vogella.com/tutorials/EclipseGradle/article.html
YMMV since I find the Gradle Plugin to be of little value so far.