Examples need to be built stand-alone and as part of Qt, so they
need a special CMakeLists.txt file that supports both use-cases.
Add an --is-example switch to pro2cmake to make it generate these special
CMakeLists.txt files.
This is basic support only and is currently still missing the necessary
find_package calls.
Change-Id: Ie770287350fb8a41e872cb0ea607923caa33073d
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>