Make the text for empty days configurable

The default is "--"; a single space makes the text invisible.

Signed-off-by: Lars Henriksen <LarsHenriksen@get2net.dk>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/calcurse.h

@@ -287,6 +287,7 @@ struct conf {
 	unsigned event_separator;
 	unsigned day_separator;
 	unsigned empty_appt_line;
+	char empty_day[BUFSIZ];
 	const char *editor;
 	const char *pager;
 	const char *mergetool;
@@ -296,6 +297,8 @@ struct conf {
 	char day_heading[BUFSIZ];	/* format for displaying heading in appts panel */
 };
 
+#define EMPTY_DAY_DEFAULT "--"
+
 /* Daemon-related configuration. */
 struct dmon_conf {
 	unsigned enable;	/* launch daemon automatically when exiting */