When fixed you can see the contribs compiling in the build output like this:
[echo] contrib: capacity-scheduler
[javac] /hadoop01/jenkins/jobs/hadoop-common/workspace/mapreduce/src/contrib/build-contrib.xml:193: warning: 'includeantruntime' was not set, defaulting to build.sysclasspath=last; set to false for repeatable builds
[javac] Compiling 12 source files to /hadoop01/jenkins/jobs/hadoop-common/workspace/mapreduce/build/contrib/capacity-scheduler/classes
[javac] Note: Some input files use or override a deprecated API.
[javac] Note: Recompile with -Xlint:deprecation for details.
[javac] Note: /hadoop01/jenkins/jobs/hadoop-common/workspace/mapreduce/src/contrib/capacity-scheduler/src/java/org/apache/hadoop/mapred/ContainerQueue.java uses unchecked or unsafe operations.
[javac] Note: Recompile with -Xlint:unchecked for details.