gradle cross-compile to an older Java version

build.gradle

// If on JDK 9+, verify project cross-compiles on its 'sourceCompatible' JVM version (see https://github.com/melix/mrjar-gradle/blob/master/jdks.gradle)
if (project.hasProperty('crossCompile')) {
    if (JavaVersion.current().java9Compatible) {
        project.afterEvaluate {
            tasks.withType(JavaCompile) {
                def version = compat(sourceCompatibility)
                project.logger.info("Configuring $name to use --release $version")
                options.compilerArgs.addAll(['--release', version])
            }
        }
    } else {
        project.logger.warn("-PcrossCompile not supported prior to JDK 9; using JDK ${JavaVersion.current()}")
    }
}

// This function converts 1.8 -> 8
static String compat(String src) {
    if (src.contains('.')) {
        src.substring(src.lastIndexOf('.')+1)
    } else {
        src
    }
}