Dependencies should be looked for the same way in all subprojects

In various cases, dependencies at qt configure step are not looked for the same way depending on the sub-projects

For example, some libraries that are found in qtbase are not found in qtwebengine when configure has pkgconfig disabled because qtwebengine only looks for them with pkgconfig method. It's a shame if we have the code to detect them in an other way in the code base.
Then, when i configure with pkgconfig detection enabled, i get other errors, because other config paths that use pkgconfig find a system libpng 1.5, while other paths that don't use pkgconfig detection find the libpng 1.6 i want to use and provided options to find. I end up with a 'successfull' Qt build where some libs need png 1.6 and some need png 1.5. Which does not work on system where only png1.6 is provided.

I don't know how hard it is to have dependency search for each lib not duplicated in various places, but i think it should be done