Hacking QTC to work with Cygwin's MinGW (paths problem)

[_PIB]

For example: stdlib.h is included from sample project "puzzle". Project compiles well with all four kits which I have (x86/x64 MinGW/Cygwin) BUT this include line is underlined with yellow as unknown file in Qt Creator editor with only one kit selected - Cygwin's MinGW.

I have found that qmake parses gcc output to learn the include paths and cygwin's GCC returned POSIX paths so I modified the mkspecs/features/toolchain.prf to fix it:
http://pastebin.com/raw/Yv0buTUg (search for cygwin)

I made diagnostic output which I see when I select different kit and I double checked the correctness of paths - they now have forward slashes and are similar to MinGW's detected paths.

Yet QTC underlines system includes as incorrect. What am I missing? Is there a cached include path set somewhere?

Example "General messages" output: http://pastebin.com/raw/RRqAVfYR

[_PIB]

Adding all include directories reported by GCC to .pro file solved the issue for now but I would still like to know a persistent solution.