Not sure, but probably you have to do a "nmake distclean" in between or it finds your license information. However, this is the opensource forum. You would need to approach the commercial, they have to able to give the proper answer.
The only thing to take into account is that any update to the Qt provided xcb library won't happen until next release of Qt (unless you're building from the latest git revision) while your system xcb will follow your distribution updates.
If you want a full static application then you have to also build all dependencies as static and since you're on Windows, you also have to take into account the Visual Studio Runtime (since you are using a Visual Studio Qt build). Note that this is nothing Qt specific.
No, you don't have to delete anything, you can use several builds of Qt in parallel without any problem. How fast your build time will be depends on the machine you have.
Developing applications and the type of builds are two completely unrelated topics.
Note that using a static build of Qt has licensing implications.
Okay... finally i solved it. Nothing to do with .pro file or PLUGIN macro. It was a basic PATH issue. qmake was searching for 'plugin' directory in /PREFIX path not in /SYSROOT/PREFIX path. I just copied all plugin directory from /SYSROOT/PREFIX to /PREFIX directory. I know this may not be the right way, but it temporarily solved my issue. If someone could suggest the correct solution please let me know.
Having a static Qt doesn't mean that all dependencies are gone. Qt might be static but unless you also link to the static version of all its dependencies, what you have is normal: OpenGL and the xcb plugin deps.
Another interesting development: I tried running on another machine with the Qt toolchain installed, and the program worked. So it appears it is still reading something from the filesystem that is not on a regular Windows machine, but I don't know what that might be. I did package all the qmldir files needed for Qt Quick to work.
@jitu059_Lalat The long and short if it are, you have two ways to do this:
Find all the dll's that your app depends on. You can do this with the tool http://www.dependencywalker.com/. Once you found all the dll's copy them into the same folder as your executable. This is now your "delivery package" you will copy the executable + dlls from PC to PC. This method is perhaps a bit easier/faster in the short term, but when you compile with different versions of Qt/Windows you may require a different set of dlls (i.e. Qt4 Dlls don't match Qt5 Dlls).
Download the Qt source code and compile it for static linking. Then when you build your code you will get a much larger executable file, but it will contain everything it needs - a standalone executable. This is perhaps a bit more work on your part, but master this and it makes deploying your project from PC to PC much easier.
Have a go at one or other method, if you get stuck then ask again. But as SGaist said, there is lots of documentation out there already : )