Mmm.... confusion in the instructions.
FOR THE FUTURE READER: the following instructions are valid for a very limited period of time. They could be invalid even tomorrow morning.
If you (the user in the future) use them and they don't work, I'll automatically assume you're dumber than a pile of rocks and you shouldn't be allowed to use (or even be near) a computer.
1) download ns-3.41
2) "cd contrib"
4) cd quic
5) git checkout release-3-41
6) cd ../.. (the ns-3 root directory)
7) ./ns3 configure --enable-example
8) try to run "./ns3 run quic-variants-comparison-bulksend" - it should work.
If 8 doesn't work, then there's an installation issue.
If 8 works, then you can either:
1) copy the scripts you want to run to the scratch folder, or
2) modify the file "contrib/quic/examples/CMakeLists.txt".
There's no need to modify any other cmake file, and you must forget about waf - we don't use it anymore.
Last but not least, mind that you might need to update the compiler, check the ns-3 installation manual for the exact requirements. Unfortunately Ubuntu 18.04 is unsupported by ns-3 (too old), so also using a newer Ubuntu is advisable.