Build-errors due to compiler version

Currently in newly created git-repo ARC does not build due to variable not defined.

Base64.cpp:132: error: 'nullptr' was not declared in this scope

Please suggest a fix, or fork the repo, create dev-branch from master, and ask for merge request to master. Release manager will port changes to the other branches.

Link to last changes in file: 4db23747

Continuous integration on your fork to build the code automatically

To turn on your own continuous integration: In .gitlab-ci.yml comment out the parts like so:

#only:
# - master@nordugrid/arc

Then, go to Settings/CI/CD/Runner Settings and enable all the runners.

Please fix all build errors before merging back into master.