re-use pre-existing builds
run "cmake . [...]" if a build already exists in the build dir. this saves the user from reproducing all the arguments he may need. on the downside, this will go utterly awry if the source dir of the existing build is a different one than known to creator. this should be a rather rare corner case, so ignore it for now. the nice solution would be a checkbox (on by default) to request this new behavior.
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