Add check to scons build to make sure git-lfs is installed

[tst-ccamp]

Many people have attempted to clone the repo without first installing git-lfs. The error that they receive upon building is confusing ("gzip: stdin: not in gzip format"). It would be better if scons would perform the check to make sure the repo was successfully cloned and print out a helpful error message and direct the developer on how to fix the problem.