BuildManager::cancel() needs to block until the step is canceled

Task-number: QTCREATORBUG-10256

Change-Id: I8e761ac072954eb1802b9b4eec415f48414b7990
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/projectexplorer/buildmanager.cpp

@@ -216,8 +216,13 @@ void BuildManager::cancel()
             return;
         d->m_canceling = true;
         d->m_watcher.cancel();
-        if (d->m_currentBuildStep->runInGuiThread())
+        if (d->m_currentBuildStep->runInGuiThread()) {
             d->m_currentBuildStep->cancel();
+            while (d->m_canceling)
+                QApplication::processEvents(QEventLoop::ExcludeUserInputEvents);
+        } else {
+            d->m_watcher.waitForFinished();
+        }
     }
 }