Problems building for older Macs

Hi folks,
I have a Macbook running OSX 10.5. I want to distribute a prolog  
executable to run on older Macs, but I have problems.
Firstly, my executable just gives 'bus error' when running on an OSX  
10.4 Mac. This is even though I supply the -mmacosx-version-min=10.3  
flag to gcc via the '-C' option of gplc when building it. I rebuilt  
Prolog itself using this flag, but that did not help. This is using  
gcc 4.0.
Secondly I tried to build an executable for PowerPC macs by including  
the '-arch ppc' flag. This built the object code OK, but when making  
the executable I got:
cc1: error: unrecognized command line option "-march=i486"
In fact the object file reports itself as being of i386 architecture  
so I would be quite prepared to believe this is impossible!
	--Jasper