Debugger: Allow pure qml debugging if no C++ debugger is configured

Don't complain e.g. about a missing cdb debugger if the user doesn't
want to debug C++ anyway.

Task-number: QTCREATORBUG-2376
Reviewed-by: hjk

src/plugins/debugger/debuggerrunner.cpp

@@ -537,6 +537,9 @@ bool DebuggerRunControl::checkDebugConfiguration(int toolChain,
 
     bool success = true;
 
+    if (!(DebuggerPlugin::instance()->activeLanguages() & CppLanguage))
+        return success;
+
     switch(toolChain) {
     case ProjectExplorer::ToolChain::GCC:
     case ProjectExplorer::ToolChain::LINUX_ICC: