Add audible bell

\a is the character for bell, and st is only marking the window as urgent
if it is not active. This patch adds an audible bell which can be disable
with bellvolume variable.

Signed-off-by: Christoph Lohmann <20h@r-36.net>

st.c

@@ -2320,6 +2320,8 @@ tputc(char *c, int len) {
 		case '\a':   /* BEL */
 			if(!(xw.state & WIN_FOCUSED))
 				xseturgency(1);
+			if (bellvolume)
+				XBell(xw.dpy, bellvolume);
 			return;
 		case '\033': /* ESC */
 			csireset();