build.d: Add condition delegate

[marler8997]

Currently when you run build.d it will re-run some of the dependencies every time. This is because some rules don't use file timestamps determine whether or not they should run. This change adds the condition property to the Dependency object that is used to determine whether or not the rule needs to be run.

This means we can still use the msg property and it will only be printed when the dependency is actually executed.

[dlang-bot]

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