无法构建S4：无法在根项目's'上找到属性'ext'

Question

我正在尝试从源代码构建S4。我已经安装了gradle并执行了“gradle wrapper”。

$ gradle wrapper
...
A problem occurred evaluating root project 's4'.
Cause: Could not find property 'ext' on root project 's4'.
...
* Exception is:
org.gradle.api.GradleScriptException: A problem occurred evaluating root project 's4'.
    at org.gradle.groovy.scripts.DefaultScriptRunnerFactory$ScriptRunnerImpl.run(DefaultScriptRunnerFactory.java:51)
...
Caused by: groovy.lang.MissingPropertyException: Could not find property 'ext' on root project 's4'.

我做错了什么？