Maven plugin should log warning if target version is not set

[GoogleCodeExporter]

Using the maven plugin of forbidden-apis and not setting either the property 
"maven.compiler.target" or the plugin parameter "targetVersion" will lead to a 
build failure because the plugin can not find the bundled resources for the 
current java version. Logging a warning if the target version is null could 
lead the user to this misconfiguration and avoid some time of research.

Original issue reported on code.google.com by jfromm...@picturesafe.de on 23 Jun 2014 at 9:49