gradle build running很久并出现"png-cruncher_19" java.lang.RuntimeException

刚开始androidstudio构建时一直卡在那里gradle build running很久，一晚上都没成功，后来通gradle控制台过发现下面的错误信息：

Exception in thread "png-cruncher_19" java.lang.RuntimeException: Timed out while waiting for slave aapt process, make sure the aapt execute at E:\tools\adt\sdk\build-tools\android-7.0\aapt.exe can run successfully (some anti-virus may block it) or try setting environment variable SLAVE_AAPT_TIMEOUT to a value bigger than 5 seconds
	at com.android.builder.png.AaptProcess.waitForReady(AaptProcess.java:108)
	at com.android.builder.png.QueuedCruncher$1.creation(QueuedCruncher.java:120)
	at com.android.builder.tasks.WorkQueue.run(WorkQueue.java:203)
	at java.lang.Thread.run(Thread.java:745)

从日志中看（try setting environment variable SLAVE_AAPT_TIMEOUT to a value bigger than 5 seconds），表示SLAVE_AAPT_TIMEOUT 的值需要大于5，于是增加系统环境变量（SLAVE_AAPT_TIMEOUT）并设置为30，再次编译就ok啦。