FastJet 3.4.0, master branch (2021.06.30.)

Upgraded the code to build FastJet 3.4.0, as part of the effort of upgrading all our projects to use this new version.

For all LCG based projects I now removed the build of FastJet, as the one provided by LCG will be "good enough" from now on. In the current setup only AnalysisBaseExternals continues building this external.

As I had to learn from the lcgcmake configuration, and then from seeing the compiler options myself as well, FastJet's AutoConfiguration does not provide optimisation flags automatically to the build anymore. 😕 So just like the LCG configuration, ours also has to now explicitly tell the FastJet build to produce optimised binaries (when needed). And while going down this rabbit hole, I had to realise that FastJetContrib was probably never receiving proper optimisation flags in our builds. 😦 Since we only used it for AnalysisBase since quite a while, this should not be as bad as if we would've done it in for the reconstruction code. Still, quite embarrassing...

This is all done in conjunction with SPI-1929, and a lengthy e-mail thread between a number of us.

Of course this should only go in right before we switch to LCG_100_ATLAS_5.

Pinging @elmsheus, @emoyse, @delsart, @cdelitzs, @mhodgkin, @goetz, @wbalunas, @sawyer, @khoo. 😉