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

It depends on which toolchain is defined for the current project.

Toolchains are defined in Tools-Options-Build & Run-Tool Chains. Most are autodetected, but you can add your own.

There is no default toolchain, so you need a project for those headers to be found in the toolchain selected for it.