Qt Creator can't find headers (says: “No such file or directory”)

Question

How can I tell Qt Creator 2.4.1 (based on Qt 4.7.4 32-bit) where to look by default for header files?

When I open a C file in Qt Creator and say

Answer 1

add INCLUDEPATH to "project.pro" file

example for raspberry pi qt projects:
# Qt Creator can't find headers (says: “No such file or directory”)
# add compile includes
INCLUDEPATH += /home/user/raspi/tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/arm-linux-gnueabihf/libc/usr/include/ \
               /home/user/raspi/tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64/arm-linux-gnueabihf/libc/usr/include/arm-linux-gnueabihf/
# pi rootfs includes
INCLUDEPATH += /home/user/raspi/sysroot/usr/include/