Fix FullScreen on Lion compile errors

Fix compile on Mac SDK < 10.7, silly initialisation
of fullScreen in MainWindow when it doesn't exist
and fixup gcconfig.pri.in definiton of GC_HAVE_LION.

src/MainWindow.cpp

@@ -137,8 +137,6 @@ MainWindow::MainWindow(const QDir &home) :
     QVariant unit = appsettings->value(this, GC_UNIT);
     useMetricUnits = (unit.toString() == "Metric");
 
-    fullScreen = NULL;
-
 #if (defined Q_OS_MAC) && (defined GC_HAVE_LION)
     fullScreen = new LionFullScreen(this);
 #endif