OperationBehavior not rebuild if source file changes

[nrother]

In 7805ee5 code was added to automatically build the *.opb if it is missing. This is very convenient, but it only checks if the * .opb file exists. It would even be more useful if it would compare the file dates and rebuild the behavior if the source file is never that the compiled one.

In the current version it somehow encourages you to remove the buildopset step from your Makefile, but then later changes to the source file are not picked up correctly.