[Development] Making sure all platform plugin library names are prefixed with "q"

[Samuel Rødal]

Hello,

currently the platform plugin library name is a bit inconsistent. In 
most cases it will have the "q" prefix, for instance libqeglfs.so, 
libqminimal.so, libqcocoa.so, etc.

However, the qnx, windows, and xcb libraries were named libqnx.so, 
libwindows.so, and libxcb.so respectively. The latter might be 
especially problematic since the system xcb library is typically also 
called libxcb.so.

The static linking support currently assumed the latter, non-prefixed 
library names, which means static linking is currently broken with any 
of the other platform plugins.

The proposal is to always prefix the library names with "q", for 
consistency and avoiding name clashes: 
https://codereview.qt-project.org/#change,42347

This doesn't affect the platform key used with the QT_QPA_PLATFORM env 
variable or the "-platform" command line argument, it will still be 
"xcb" in the xcb platform plugin case for instance.

The only risk I can see is that stale plugins will be lying around in 
the build directories of people doing incremental builds. That might 
mean a few people could end up using a slightly older plugin version 
without newer fixes for a short while. When / if the change gets merged 
I will send another mail reminding people to delete their 
plugins/platforms directory before updating to the latest stable.

--
Samuel