work around no convertRectToScreen on < 10.7

Of note, it looks a lot like the new convertPointToScreen is just the old convertBaseToScreen
in new clothes... what goes around comes around, I guess

--- src/gui/menu_osx.mm.orig    2020-09-02 12:14:28.000000000 -0700
+++ src/gui/menu_osx.mm 2020-09-02 12:16:38.000000000 -0700
@@ -511,7 +511,16 @@
 // NTS: This did not appear until Mojave, and some followers on Github prefer to compile for somewhat older versions of OS X
 //      NSPoint pt = [wnd convertPointToScreen:NSMakePoint(rct.size.width / 2, rct.size.height / 2)];
 // NTS: convertRectToScreen however is documented to exist since 10.7, unless Apple got that wrong too...
-        NSPoint pt = [wnd convertRectToScreen:NSMakeRect(rct.size.width / 2, rct.size.height / 2, 0, 0)].origin; /* x,y,w,h */
+        NSPoint pt = {0,0};
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= 1070
+        if ([wnd respondsToSelector:@selector(convertRectToScreen:)]) {
+             pt = [wnd convertRectToScreen:NSMakeRect(rct.size.width / 2, rct.size.height / 2, 0, 0)].origin; /* x,y,w,h */
+        } else {
+             pt = [wnd convertBaseToScreen:NSMakeRect(rct.size.width / 2, rct.size.height / 2, 0, 0).origin]; /* x,y,w,h */
+        }
+#else
+        pt = [wnd convertBaseToScreen:NSMakeRect(rct.size.width / 2, rct.size.height / 2, 0, 0).origin]; /* x,y,w,h */
+#endif
 
         {
             /* Eugh this ugliness wouldn't be necessary if we didn't have to fudge relative to primary display. */