An extention for Qt's configure helping to test a toolchain for C++ compatibility

As we will integrate more C++20 and newer features over time, we should provide a tool to Qt user which would help to test whether a toolchain would be capable to supporting all C++ features used in a given version of Qt.

Why is configure suggested here?

• it is a bootstrap tool designed to be run from scratch
• it can detect what the underlying system can support
• it can show results without having to build Qt
• it takes less time and skills to run it. So it can also be used by hardware and toolchain vendors who lack knowledge and time to deal with details of a full Qt build.

It is clear that the results of that tool cannot be seen as a 100% guarantee that a given toolchain will compile Qt successfully and it will run as expected. Still, it has a big value with its initial indications.