[quartz] Fix build on macOS versions before 10.14.

shouldInheritContentScale was introduced in 10.14.

gdk/quartz/GdkQuartzView.c

@@ -526,6 +526,7 @@ copy_rectangle_argb32 (cairo_surface_t *dest, cairo_surface_t *source,
 
 }
 
+#if MAC_OS_X_VERSION_MIN_REQUIRED >= 101400
 -(BOOL)layer:(CALayer*) layer shouldInheritContentsScale: (CGFloat)scale fromWindow: (NSWindow *) window
 {
   if (layer == self.layer && window == self.window)
@@ -535,6 +536,7 @@ copy_rectangle_argb32 (cairo_surface_t *dest, cairo_surface_t *source,
     }
   return YES;
 }
+#endif
 
 -(void)setFrame: (NSRect)frame
 {

gdk/quartz/GdkQuartzView.h

@@ -34,7 +34,11 @@
 #define GIC_FILTER_PASSTHRU	0
 #define GIC_FILTER_FILTERED	1
 
+#if MAC_OS_X_VERSION_MIN_REQUIRED < 101400
+@interface GdkQuartzView : NSView <NSTextInputClient>
+#else
 @interface GdkQuartzView : NSView <NSTextInputClient, NSViewLayerContentScaleDelegate>
+#endif
 {
   GdkWindow *gdk_window;
   NSTrackingRectTag trackingRect;