Add idea: Checksum of preprocessed output as change detection

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+- Checksum of preprocessed output as change detection [idea]
+
+If instead of a timestamp (or, really, in addition to timestamp) we hash the
+preprocessor output, then we can detect (and avoid updating on) whitespace-
+only changes (e.g., edited a comment).
+
+The big question will be how to reuse this output if we do need to recompile.
+
+Also, this may fit nicely with the build slave idea.