Interesting idea, I hadn't thought of that before. This only increases the complexity of the Makefile though, and for not much reason. Hashing can help in other circumstances (such as possibly skipping a linking step if the object files don't change), and it would really be much cleaner to have it as part of the build system. The last time I played around with generating code through make, it got real ugly real fast, and I don't think your 'install' trick will help much in that regard...
There's actually a good reason to do this, but it is an edge case. A timestamp-based build system can do a no-op check with very little I/O. A hash-based build system has to read all of the file contents in order to determine that nothing has changed. Depending on the latency and bandwidth of your storage, this can make a big difference in incremental builds.
I was more talking about the need to implement this layer yourself, when it should be taken care of by the build system (though I don't know of any build systems that can easily implement your suggestion, since it has to somehow redirect the output of a code-generation step to another file). Agreed that there are performance differences, though in my experience hashing is quite acceptable even for large projects if used reasonably (only hashing inputs to build steps if they are also outputs of another build step).
