Fix GNUstep build

* src/nsterm.h [NS_IMPL_GNUSTEP]: Add typedefs for Cocoa-only types.
(NSWindowStyleMaskUtilityWindow): #define to NSUtilityWindowMask in
GNUstep and old versions of macOS.
* src/nsfns.m (ns-set-mouse-absolute-pixel-position): Function only
works in cocoa, not GNUstep.

src/nsfns.m

@@ -3046,6 +3046,9 @@ value is a list of the form (LEFT, TOP, RIGHT, BOTTOM).  All values are
 \(0, 0) of the selected frame's display.  */)
        (Lisp_Object x, Lisp_Object y)
 {
+#ifdef NS_IMPL_COCOA
+  /* GNUstep doesn't support CGWarpMouseCursorPosition, so none of
+     this will work. */
   struct frame *f = SELECTED_FRAME ();
   EmacsView *view = FRAME_NS_VIEW (f);
   NSScreen *screen = [[view window] screen];
@@ -3072,6 +3075,7 @@ value is a list of the form (LEFT, TOP, RIGHT, BOTTOM).  All values are
 
   CGPoint mouse_pos = CGPointMake(mouse_x, mouse_y);
   CGWarpMouseCursorPosition (mouse_pos);
+#endif /* NS_IMPL_COCOA */
 
   return Qnil;
 }

src/nsterm.h

@@ -62,6 +62,13 @@ typedef CGFloat EmacsCGFloat;
 typedef float EmacsCGFloat;
 #endif
 
+/* FIXME: It looks as though instancetype will be supported in GNUstep
+   at some point, but I'm not sure what version. */
+#ifdef NS_IMPL_GNUSTEP
+typedef id instancetype;
+typedef int NSWindowStyleMask;
+#endif
+
 /* ==========================================================================
 
    Trace support
@@ -1290,6 +1297,7 @@ extern char gnustep_base_version[];  /* version tracking */
 #define NSWindowStyleMaskMiniaturizable    NSMiniaturizableWindowMask
 #define NSWindowStyleMaskResizable         NSResizableWindowMask
 #define NSWindowStyleMaskTitled            NSTitledWindowMask
+#define NSWindowStyleMaskUtilityWindow     NSUtilityWindowMask
 #define NSAlertStyleCritical               NSCriticalAlertStyle
 #define NSControlSizeRegular               NSRegularControlSize
 #endif