Updates containing: - updates to make AnalysisBaseExternals build out of the box on top of macOS High Sierra with Xcode 9.0; - the ability to override (from the command line) which LCG version the LCG using external projects should build against; - a fix for building lwtnn on platforms providing an acceptable version of Boost, like Ubuntu 16.04; - a small fix for BAT to build correctly against LCG_91.