The class javax.xml.XMLConstants is twice in the classpath:
1. From the Java Runtime (from 1.6 or 1.7)
2. From the maven dependency
stax/stax-api-1.0.1 (http://stax.codehaus.org/)

Number 2 does not define the constant FEATURE_SECURE_PROCESSING, which is used in jenkins.util.xml.XMLUtils. It seems that at least Eclipse gives preference to the codehaus variant so this leads to a compilation error.

Interestingly, there is one more artifact with the name "jstax-api", but it doesn't contain javax.xml.XMLConstants.

See also: https://issues.jenkins-ci.org/browse/JENKINS-2547

Source and target version of Maven are at 1.7 - I guess Java 1.5 support is not an issue any more.