Skip if output is younger than input

Just like the compiler plugin works, the exec plugin should skip execution in case the output is younger than the input.

As the exec plugin per se is unaware what the input and output to the execution is, a smart solution could be to configure it with feasible default:

• holds patterns for input files, and defaults to ${project.build.sourceDirectory}.
• holds patterns for output files, and defaults to ${project.build.outputDirectory}.

As a result:

• When working with a plain Java project, no configuration is needed; the performance optization is granted for free.
• When backwards compatibility is wanted, deactivate the optimization using empty and .
• In all other case, provide feasible regex patters for and to safely detect only those files that play a role.

Start by examining the exec plugin configuration and the existing compiler plugin behavior referenced in the issue. Determine how input and output patterns should default to Maven source and output directories, how timestamp-based skipping should work, and how empty inputs/outputs preserve compatibility; done means the behavior is covered for plain Java projects and custom patterns.