Fix feature macros for FreeBSD

On FreeBSD, snprintf and vsnprintf are visible only if
_XOPEN_SOURCE >= 600.

client/pool-buffer.c

@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 500
+#define _XOPEN_SOURCE 600
 #include <assert.h>
 #include <cairo/cairo.h>
 #include <fcntl.h>

sway/commands.c

@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 500
+#define _XOPEN_SOURCE 600
 #include <ctype.h>
 #include <stdarg.h>
 #include <stdlib.h>

sway/commands/bar.c

@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 500
+#define _XOPEN_SOURCE 600
 #include <string.h>
 #include <strings.h>
 #include <wlr/util/log.h>

sway/commands/output/background.c

@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 500
+#define _XOPEN_SOURCE 600
 #include <libgen.h>
 #include <strings.h>
 #include <unistd.h>

sway/tree/workspace.c

@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 500
+#define _XOPEN_SOURCE 600
 #include <ctype.h>
 #include <limits.h>
 #include <stdbool.h>

swaybar/ipc.c

@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 500
+#define _XOPEN_SOURCE 600
 #include <limits.h>
 #include <string.h>
 #include <strings.h>