QmlProject: Disable debug action if qmlobserver cannot be built

QmlObserver right now requires 4.7.1 minimum. There's no gain in
telling the user that he needs qmlobserver to debug, if it can't be
build with 4.7.0 anyway.

Reviewed-by: Christiaan Janssen

src/plugins/qmlprojectmanager/qmlprojectruncontrol.cpp

@@ -43,6 +43,8 @@
 #include <debugger/debuggeruiswitcher.h>
 #include <debugger/debuggerengine.h>
 #include <qmljsinspector/qmljsinspectorconstants.h>
+#include <qt4projectmanager/qtversionmanager.h>
+#include <qt4projectmanager/qmlobservertool.h>
 #include <qt4projectmanager/qt4projectmanagerconstants.h>
 
 #include <QApplication>
@@ -148,9 +150,18 @@ bool QmlRunControlFactory::canRun(RunConfiguration *runConfiguration,
     } else {
         bool qmlDebugSupportInstalled = Debugger::DebuggerUISwitcher::instance()->supportedLanguages()
                                         & Debugger::QmlLanguage;
-        // don't check for qmlobserver already here because we can't update the run buttons
-        // if it has been built in the meantime
-        return (config != 0) && qmlDebugSupportInstalled;
+
+        if (config && qmlDebugSupportInstalled) {
+            if (!config->observerPath().isEmpty()) {
+                return true;
+            }
+
+            if (config->qtVersion() && Qt4ProjectManager::QmlObserverTool::canBuild(config->qtVersion())) {
+                return true;
+            } else {
+                return false;
+            }
+        }
     }
 
     return false;