These two alternative patches implement root-cmd without modifying setupWrapper, with less code;

cabal repo:
Thu Mar 6 16:53:16 CET 2008 Andrea Vezzosi <sanzhiyan@gmail.com>
* move inDir to Distribution.Simple.Utils

cabal-install repo:
Thu Mar 6 17:08:16 CET 2008 Andrea Vezzosi <sanzhiyan@gmail.com>
* Implement CLI flag --root-cmd for 'cabal install' as in #202
It also adds the internal flag --only so we can avoid building a setup when build-type == Simple.