Adjust FLEX_AT_LEAST_IOS11_SDK to work with -Wexpansion-to-defined

Classes/ExplorerInterface/FLEXExplorerViewController.m

@@ -695,7 +695,7 @@ typedef NS_ENUM(NSUInteger, FLEXExplorerMode) {
 - (CGRect)viewSafeArea
 {
     CGRect safeArea = self.view.bounds;
-#if FLEX_AT_LEAST_IOS11_SDK
+#ifdef FLEX_AT_LEAST_IOS11_SDK
     if (@available(iOS 11, *)) {
         safeArea = UIEdgeInsetsInsetRect(self.view.bounds, self.view.safeAreaInsets);
     }
@@ -703,7 +703,7 @@ typedef NS_ENUM(NSUInteger, FLEXExplorerMode) {
     return safeArea;
 }
 
-#if FLEX_AT_LEAST_IOS11_SDK
+#ifdef FLEX_AT_LEAST_IOS11_SDK
 - (void)viewSafeAreaInsetsDidChange
 {
   if (@available(iOS 11, *)) {

Classes/Toolbar/FLEXExplorerToolbar.m

@@ -261,7 +261,7 @@
 - (CGRect)safeArea
 {
   CGRect safeArea = self.bounds;
-#if FLEX_AT_LEAST_IOS11_SDK
+#ifdef FLEX_AT_LEAST_IOS11_SDK
   if (@available(iOS 11, *)) {
     safeArea = UIEdgeInsetsInsetRect(self.bounds, self.safeAreaInsets);
   }

Classes/Utility/FLEXUtility.h

@@ -14,7 +14,9 @@
 
 #define FLEXFloor(x) (floor([[UIScreen mainScreen] scale] * (x)) / [[UIScreen mainScreen] scale])
 
-#define FLEX_AT_LEAST_IOS11_SDK defined(__IPHONE_11_0) && (__IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_11_0)
+#if defined(__IPHONE_11_0) && (__IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_11_0)
+#define FLEX_AT_LEAST_IOS11_SDK
+#endif
 
 @interface FLEXUtility : NSObject