[CMake] How to build only when file contents are actually changed?

[Peng Yu]

> Again, using ccache solves this much more elegantly. And calling md5sum
> twice is also not very nice...

I'm not sure ccache replaces hash. My understanding is that ccache
speed up individual compilation, but all the targets that depends on
it are still compiled. With hash, a file is checked first whether it
should be recompiled, if not, it will not be recompiled, and all the
targets that depend on it will not be compiled either. I'm not with a
machine that I can check ccache as this moment. Please correct me if
I'm wrong.

The following compares a few features. But since it is from waf, it
might be biased in favor of waf. Nevertheless, there are some features
that available in other tools missing in cmake.

http://code.google.com/p/waf/wiki/WafAndOtherBuildSystems

-- 
Regards,
Peng