Especially when using CMake based projects in combination with GTest
we might have lots of build targets defined inside the same
'project file'. Overwriting the already assigned content is purely
wrong. Instead handle each found build system target on its own.

Task-number: QTCREATORBUG-17882
Task-number: QTCREATORBUG-18932
Change-Id: I3297a9e1da8ae4186fd4267e3485368b1a059314
Reviewed-by: David Schulz <david.schulz@qt.io>