亲测解决：The following dependencies on the compile classpath are found to contain annotation processor.

完整的错误信息是：

Annotation processors must be explicitly declared now.  The following dependencies on the compile classpath are found to contain annotation processor.  Please add them to the annotationProcessor configuration.
  - arouter-compiler-1.1.4.jar (com.alibaba:arouter-compiler:1.1.4)
  - auto-service-1.0-rc2.jar (com.google.auto.service:auto-service:1.0-rc2)
Alternatively, set android.defaultConfig.javaCompileOptions.annotationProcessorOptions.includeCompileClasspath = true to continue with previous behavior.  Note that this option is deprecated and will be removed in the future.
See https://developer.android.com/r/tools/annotation-processor-error-message.html for more details.

项目中，我用config.gradle统一系统配置信息和第三方库，如下图：

 

在使用的build.gradle中：

api rootProject.ext.dependencies.arouter_compiler

解决方法（用错了引用方式）：

  annotationProcessor rootProject.ext.dependencies.arouter_compiler