I have found a reason, why LD_LIBRARY_PATH, CPLUS_INCLUDE_PATH and LIBRARY_PATH variables were ignored by QtCreator: I use KDE and it does not load the .bashrc in the session, but the bash does. So in KDE when you open Konsole - your environment is populated from .bashrc, but other apps, not launched from console - have different environment. So for me the solution was to create $HOME/.kde/env/user.sh file like this:
if [ -f ~/.username.bashrc ]; then
~/.username.bashrc contains my environment variables.