I'm developing an application using QT. And before installed the last version of SDK I didn't have any problem, compiling my application.

I try to compile a test application (hello.cpp) and it happens the same, it gives me an error:

[sbox-FREMANTLE_X86: ~/hello] > make
g++ -c -pipe -O2 -Wall -W -D_REENTRANT -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/targets/FREMANTLE_X86/usr/share/qt4/mkspecs/linux-g++ -I. -I/targets/FREMANTLE_X86/usr/include/QtCore -I/targets/FREMANTLE_X86/usr/include/QtGui -I/targets/FREMANTLE_X86/usr/include -I. -I. -o hello.o hello.cpp
g++ -Wl,-O1 -Wl,-rpath,/usr/lib -o hello hello.o -L/usr/lib -lQtGui -L/usr/lib -L/usr/X11R6/lib -lQtCore -lpthread
/scratchbox/compilers/cs2007q3-glibc2.5-i486/bin/../lib/gcc/i486-pc-linux-gnu/4.2.1/../../../../i486-pc-linux-gnu/bin/ld: skipping incompatible /usr/lib/libQtGui.so when searching for -lQtGui
/scratchbox/compilers/cs2007q3-glibc2.5-i486/bin/../lib/gcc/i486-pc-linux-gnu/4.2.1/../../../../i486-pc-linux-gnu/bin/ld: skipping incompatible /usr/lib/libQtGui.so when searching for -lQtGui
/scratchbox/compilers/cs2007q3-glibc2.5-i486/bin/../lib/gcc/i486-pc-linux-gnu/4.2.1/../../../../i486-pc-linux-gnu/bin/ld: skipping incompatible /usr/lib/libQtGui.so when searching for -lQtGui
/scratchbox/compilers/cs2007q3-glibc2.5-i486/bin/../lib/gcc/i486-pc-linux-gnu/4.2.1/../../../../i486-pc-linux-gnu/bin/ld: skipping incompatible /usr/lib/libQtGui.so when searching for -lQtGui
/scratchbox/compilers/cs2007q3-glibc2.5-i486/bin/../lib/gcc/i486-pc-linux-gnu/4.2.1/../../../../i486-pc-linux-gnu/bin/ld: cannot find -lQtGui
collect2: ld returned 1 exit status
make: *** [hello] Error 1

Anybody have any idea how to fix this problem?

Thank you

Some classes have had their filenames changed case-wise (eg. QtSTUFF -> QtStuff) from 4.5 to 4.6. I can't remember which but try checking that.

Have you regenerated the makefile using qmake?

It's a complete new project. I mean, I copy hello.cpp from a tutorial, and then qmake -project and qmake ... nothing else ... And I reinstall sdk ... without delete previous one ...

It seems you got your Qt versions mixed up.
Perhaps you should delete the old SDK before installing a new one.