use __progname on BSD.

Index: clients/desktop-shell.c
--- clients/desktop-shell.c.orig
+++ clients/desktop-shell.c
@@ -40,6 +40,8 @@
 #include <time.h>
 #include <assert.h>
 
+extern char *__progname;
+
 #include <wayland-client.h>
 
 #include <libweston/config-parser.h>
@@ -1253,7 +1255,7 @@ background_create(struct desktop *desktop, struct outp
 	weston_config_section_get_string(s, "background-type",
 					 &type, "tile");
 	if (type == NULL) {
-		fprintf(stderr, "%s: out of memory\n", program_invocation_short_name);
+		fprintf(stderr, "%s: out of memory\n", __progname);
 		exit(EXIT_FAILURE);
 	}
 
