I have now successfully compiled Lazarus 1.2 from source. It works great even on an older PPC-based Mac (as it does on Windows and Intel-based Macs). Thanks again. 
Which system and which version of fpc?
MiSchi
It is FPC 2.6.2 and Mac OS X 10.5 (Leopard).
I use Lazarus for years, therefore I had previously installed Free Pascal and Lazarus 1.0.14 (via the regular installer from SourceForge). Then I downloaded the Lazarus sources from
http://sourceforge.net/projects/lazarus/files/Lazarus%20Zip%20_%20GZip/, changed the working directory of the shell to the unpacked folder with the sources and entered
make clean bigide
This command compiles and builds Lazarus 1.2 for PPC (depending from the speed of your machine it may take up to 10 minutes).
The only problem was that the bundle file was still for Intel Macs, so that Lazarus didn't start yet. I didn't find out how to change this in an elegant way, therefore I replaced it with the bundle file of Lazarus 1.0.14 for PPC but placed the file "Info.plist" from the new Intel bundle file into the Contents folder of the PPC bundle file (in order to have the current version string). Obviously, this isn't the normal solution, but I didn't find any documentation to replace the bundle via shell commands.
Anyway, after these steps, Lazarus 1.2 runs flawlessly on my G5 machine.