Gradle is used as a build tool for this project, so you have to install it. Of course, JDK is also required.
Execute: ./gradlew buildPlugin. The first run takes a long time as Gradle needs to download the whole IntelliJ SDK.
After the command executed successfully, a .zip file with the plugin will be available in the build/distributions directory.
Execute: ./gradlew test.
Execute: ./gradlew runIdea. During the first run you will get an error, something like 'PHP plugin is required...' Install the required plugin manually and execute the command again.