Building QT Programs for Mac OS X 10.5

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 peered inside the makefile, and confirmed that it includes a line:
export MACOSX_DEPLOYMENT_TARGET = 10.4

I built QT static with the following configure:
./configure -universal -sdk /Developer/SDKs/MacOSX10.5.sdk/ -opensource -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.

Is there anything else I need to be doing to get a QT application working on OS X 10.5? Does anyone have any idea why this is crashing?

It's crashing with this message:

Dyld Error Message:
Symbol not found: __ZSt16__ostream_insertIcSt11char_traitsIcEERSt13b asic_ostreamIT_T0_ES6_PKS3_i
Referenced from: /Applications/MyApp.app/Contents/MacOS/MyApp
Expected in: /usr/lib/libstdc++.6.dylib