Provide g_utf8_make_valid if not available

See #3415 for licensing discussion.

Fixes Airblader/i3#236

include/libi3.h

@@ -320,6 +320,11 @@ int ipc_recv_message(int sockfd, uint32_t *message_type,
  */
 void fake_configure_notify(xcb_connection_t *conn, xcb_rectangle_t r, xcb_window_t window, int border_width);
 
+#define HAS_G_UTF8_MAKE_VALID GLIB_CHECK_VERSION(2, 52, 0)
+#if !HAS_G_UTF8_MAKE_VALID
+gchar *g_utf8_make_valid(const gchar *str, gssize len);
+#endif
+
 /**
  * Returns the colorpixel to use for the given hex color (think of HTML). Only
  * works for true-color (vast majority of cases) at the moment, avoiding a