patch 7.4.757
Problem:    Cannot detect the background color of a terminal.
Solution:   Add T_RBG to request the background color if possible. (Lubomir
            Rintel)
diff --git a/src/proto/term.pro b/src/proto/term.pro
index b3d0df3..f0c2a01 100644
--- a/src/proto/term.pro
+++ b/src/proto/term.pro
@@ -36,6 +36,7 @@
 void stoptermcap __ARGS((void));
 void may_req_termresponse __ARGS((void));
 void may_req_ambiguous_char_width __ARGS((void));
+void may_req_bg_color __ARGS((void));
 int swapping_screen __ARGS((void));
 void setmouse __ARGS((void));
 int mouse_has __ARGS((int c));