After fighting for a long time with Builder who launched itself without displaying windows, and after waiting for the time necessary to download all the dependances, I came across the following error:
Program ../troll/gjspack/bin/gjspack found: NO
../src/meson.build:21:10: ERROR: Program '../troll/gjspack/bin/gjspack' not found or not executable
Unfortunately I have already devoted too much time to this problem for today.