QLayout: use qEnvironmentVariableIntValue()

It doesn't allocate memory, so cannot throw and is a lot faster
than qgetenv().

Change-Id: I355cd4ce9d0e1a56f1b40a340953d6a4b4a58629
Reviewed-by: hjk <hjk@qt.io>

src/widgets/kernel/qlayout.cpp

@@ -810,7 +810,7 @@ static bool layoutDebug()
 {
     static int checked_env = -1;
     if(checked_env == -1)
-        checked_env = !!qgetenv("QT_LAYOUT_DEBUG").toInt();
+        checked_env = !!qEnvironmentVariableIntValue("QT_LAYOUT_DEBUG");
 
     return checked_env;
 }