integrating make and git

I have an idea about integrating make with git, and I'm wondering if it is a reasonable thing to do.

First of all, I am under the impression that git can quickly compute a hash of a directory and its contents. Is that correct?

If so, suppose you using git to manage revision control of a project which has some components like 'lib1', 'lib2', etc. Typically you would perform something like: make clean; make all and 'make all' would perform 'make lib1' and 'make lib2'. When checking out a different revision of the project you would have to perform another 'make clean' before 'make all' since you aren't sure of what's changed and the timestamps of the derived files will be more recent than the timestamps of the source files.

Now suppose that making 'lib1' only depends on the source code in a certain directory. The idea is to associate the hash of the source directory for lib1 with its the derived files. Make can check this to determine if the component really needs to be rebuilt. Then as you move around in the repository you can avoid rebuilding components that haven't changed.

Good, bad, ugly?