Only show sendPrimary on X11 platforms

Primary selection is inherently a X11 concept so there is no point
showing the settings related to it on other platforms.
diff --git a/vncviewer/OptionsDialog.cxx b/vncviewer/OptionsDialog.cxx
index 3c8fcb4..f112a52 100644
--- a/vncviewer/OptionsDialog.cxx
+++ b/vncviewer/OptionsDialog.cxx
@@ -264,7 +264,9 @@
   viewOnlyCheckbox->value(viewOnly);
   acceptClipboardCheckbox->value(acceptClipboard);
   sendClipboardCheckbox->value(sendClipboard);
+#if !defined(WIN32) && !defined(__APPLE__)
   sendPrimaryCheckbox->value(sendPrimary);
+#endif
   systemKeysCheckbox->value(fullscreenSystemKeys);
 
   menuKeyChoice->value(0);
@@ -372,7 +374,9 @@
   viewOnly.setParam(viewOnlyCheckbox->value());
   acceptClipboard.setParam(acceptClipboardCheckbox->value());
   sendClipboard.setParam(sendClipboardCheckbox->value());
+#if !defined(WIN32) && !defined(__APPLE__)
   sendPrimary.setParam(sendPrimaryCheckbox->value());
+#endif
   fullscreenSystemKeys.setParam(systemKeysCheckbox->value());
 
   if (menuKeyChoice->value() == 0)
@@ -698,11 +702,13 @@
                                                        _("Send clipboard to server")));
   ty += CHECK_HEIGHT + TIGHT_MARGIN;
 
+#if !defined(WIN32) && !defined(__APPLE__)
   sendPrimaryCheckbox = new Fl_Check_Button(LBLRIGHT(tx, ty,
                                                      CHECK_MIN_WIDTH,
                                                      CHECK_HEIGHT,
                                                      _("Send primary selection and cut buffer as clipboard")));
   ty += CHECK_HEIGHT + TIGHT_MARGIN;
+#endif
 
   systemKeysCheckbox = new Fl_Check_Button(LBLRIGHT(tx, ty,
                                                     CHECK_MIN_WIDTH,