Making application run on any architecture of Mac

I'm running Mac OS X 10.6 on my development machine and am building a QT application that I would like to work on 10.5 (and 10.4) as well as 10.6. My application has both a library component and a gui component, each as their own QT project with dependencies.

I have followed the instructions of the QT deployment guide. In both .pro files I have added:

CONFIG += x86 ppc
QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.4


I built QT static with the following configure: ./configure -universal -sdk /Developer/SDKs/MacOSX10.6.sdk/ -static -debug-and-release

I compiled my application in QT Creator, and the resultant application runs flawlessly on 10.6 machines, but crashes on 10.5 machines.

Please tell me how I should build Qt staticaly in order to run on any architecture............

Thanks