Add a define for whether mouse is enabled by default (still off).

options-table.c

@@ -956,7 +956,7 @@ const struct options_table_entry options_table[] = {
 	{ .name = "mouse",
 	  .type = OPTIONS_TABLE_FLAG,
 	  .scope = OPTIONS_TABLE_SESSION,
-	  .default_num = 0,
+	  .default_num = TMUX_MOUSE,
 	  .text = "Whether the mouse is recognised and mouse key bindings are "
 		  "executed. "
 		  "Applications inside panes can use the mouse even when 'off'."

tmux.h

@@ -90,6 +90,9 @@ struct winlink;
 #ifndef TMUX_TERM
 #define TMUX_TERM "screen"
 #endif
+#ifndef TMUX_MOUSE
+#define TMUX_MOUSE 0
+#endif
 
 /* Minimum and maximum layout cell size, NOT including border lines. */
 #define PANE_MINIMUM 1