LCG Version Selection Update, 1.0 branch (2017.10.02.)

This is to make it possible to override which version of LCG the externals projects should be using. In case we want to try building the code against older or newer versions.

@fwinkl, the idea is that we could update Build/AtlasBuildScripts/build_atlasexternals.sh to add

-DLCG_VERSION_NUMBER:STRING=87

to the cmake command in it. This way 21.1 could pick up all the latest CMake updates from the 1.0 branch, while still sticking to LCG_87.

While this update itself is definitely a good one, we could discuss on the MR how you think we should go about updating 21.1. (The only other option I see is to create a 1.0.X.1 tag in which we downgrade the LCG version of AthenaExternals. But I think this approach is much better, and more maintainable.)

P.S. The BAT update is just something I noticed while trying to build the analysis projects against LCG_91. Apparently the dictionaries we built for BAT so far were only working "out of luck".