more accurately test if cmake already installed

(cherry picked from commit 9053936061ee579116e859db5deafa50ac3ce2d0)
update scons to from 2.1.0 to 2.3.4
Merge pull request #131 from PatrickBM/lib_checks

minor updates
add rat-5.0.1
Compile w/ 8 threads, comment out snoing auto-updates.
change rat-dev to expect root6
Add new root6 object & RootDev class
clone my fork of rat, not the main snoplus repo
enable only the usual (subset) of root options
fix typo in GEANT4 build flag