@techee you specify the prefix when you initialize the build directory, see https://github.com/geany/geany/pull/2761#issuecomment-809698447
The error looks like g++ is using the wrong standard, @kugel- hasn't specified it and I guess all the rest of us are using a g++ that defaults to at least c++11. In fact the new Julia lexer requires c++17 so you will need a compiler capable of that (see recent changes to travis distro version on geany-plugins to get new enough compilers).