A_ITALIC was only added to curses in 2013.

Index: pager/display.c
--- pager/display.c.orig
+++ pager/display.c
@@ -217,6 +217,7 @@ static void resolve_color(struct MuttWindow *win, stru
     else
       color.attrs |= A_UNDERLINE;
   }
+#ifdef A_ITALIC
   else if (special & A_ITALIC)
   {
     if (simple_color_is_set(MT_COLOR_ITALIC) && !search)
@@ -224,6 +225,7 @@ static void resolve_color(struct MuttWindow *win, stru
     else
       color.attrs |= A_ITALIC;
   }
+#endif
   else if (ansi->attr_color)
   {
     color = *ansi->attr_color;