Provide pkg-config .pc files

Of the applications that I know that could compile with Qt 6 but won't on macOS because they require .pc files, gnuplot and octave are two examples.

On macOS, I believe the includes are the same as on every other target, and frameworks need to use -F and -framework options. I don't know enough of cmake and the complex Qt build system to provide a patch, but I can help in testing.

Nobody did the work. There are extra difficulties to handle there, like frameworks.

On macOS, there are no .pc files installed by a Qt build from source. Is there a reason for that? It would be interesting to have them.

I stumbled upon Qt6 not providing pkgconfig files for static builds and found this bug. @Martin Reboredo, I think listing library file paths in the pkgconfig file for static builds could be okay. It is unusual to list library file paths rather than the typical `-L/path/to/dir -llibrary name` in pkgconfig files, but Qt5's pkgconfig files do this for static builds.

I am working on linking to Qt with Cargo and pkgconfig files are needed for that. I am using Qt from vcpkg on macOS and Windows, which defaults to static linking on macOS.

That would be my intention, but the change has not been tested yet (to ensure it doesn't break anything) with all qt repos in our CI. This due to CI load though, not the change itself.