patch 8.2.4694: avoidance of #elif causes more preproc nesting

Problem:    Avoidance of #elif causes more preproc nesting.
Solution:   Use #elif where it is useful. (Ozaki Kiichi, closes #10081)
diff --git a/src/optionstr.c b/src/optionstr.c
index 5c3f76d..09e6d8a 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -949,14 +949,12 @@
 		|| check_opt_strings(p_wak, p_wak_values, FALSE) != OK)
 	    errmsg = e_invalid_argument;
 # ifdef FEAT_MENU
-#  ifdef FEAT_GUI_MOTIF
+#  if defined(FEAT_GUI_MOTIF)
 	else if (gui.in_use)
 	    gui_motif_set_mnemonics(p_wak[0] == 'y' || p_wak[0] == 'm');
-#  else
-#   ifdef FEAT_GUI_GTK
+#  elif defined(FEAT_GUI_GTK)
 	else if (gui.in_use)
 	    gui_gtk_set_mnemonics(p_wak[0] == 'y' || p_wak[0] == 'm');
-#   endif
 #  endif
 # endif
     }