After installing the lazarus and fpc packages and starting the IDE, it couldn't find the fpc source path. So I downloaded the fpc sources (from ftp://ftp.freepascal.org/pub/fpc/dist/3.0.2/source/fpcbuild-3.0.2.tar.gz as the Arch package suggested) and it didn't complain any more. But (I don't know whether this has to do something with that) it still doesn't compile anything and just fails with the following message:
Kompiliere Package FCL 1.0.1: Exit code 256, Fehler: 1 registerfcl.pas(45,22) Fatal: Kann process nicht finden verwendet von RegisterFCL von Package FCL.