Fixed unchanged sources check

[AndreaBaroncelli]

When attempting to use flag skipSourcesUnchanged in a code base that generates mapping code via MapStruct, I realized the flag is overworking, in that changed source files too were skipped. The issue seems to reside in lastModified check being polluted by file .maven-processor-source-files.txt, that is rewritten in method areSourceFilesSameAsPreviousRun (which seems to be part of the problem, as you would not expect this method to be in charge of rewriting the file, which should instead be generated at the very end) : as a consequence, its lastModified date is inevitably the most recent one, hence it comes after all source files' ones, which results in method isSourcesUnchanged always returning true.

I wonder whether another possible approach could be to compare maxSourceDate with minOutputDate, instead that maxOutputDate.