Since I use to forget things, I decided create my own blog-entry to solve compilation-issues with Qt. Stolen straight from datafreak.eu :
Basic XLib functionality test failed! You might need to modify the include and library search paths by editing QMAKE_INCDIR_X11 and QMAKE_LIBDIR_X11 in \ [...]/qt-all-opensource-src-4.5.2/mkspecs/linux-g++-64.
apt-get install libxext-dev
yum install libX11-devel libXext-devel