Code model does not know about sysroot that is used by default by build system

QMake, CMake and Qbs all automatically set a -isysroot when building (on macOS at least).
But the code model does not get this sysroot set.

This is especially problematic if the compiler is clang from a Xcode installation, i.e. when using /Applications/Xcode.app/......./clang++ because that actually requires a sysroot setting to find system headers. (/usr/bin/clang++ seems to automatically assume a sysroot from the MacOS SDK.)

In 4.4 we even have autodetected tool chains (Apple Clang (<arch>)) which are affected by this.