Create a profile called "java-6" for plugins that require at least Java 6 to build. It is triggered automatically based on the JDK that is being used.
Move maven-pmd-plugin to the "java-6" profile.

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1611443 13f79535-47bb-0310-9956-ffa450edef68