Display the preamble in Linux only if notify-send is installed (#401)

src/main.rs

@@ -88,7 +88,7 @@ fn run() -> Result<()> {
 
     #[cfg(target_os = "linux")]
     {
-        if config.display_preamble() && !config.skip_notify() {
+        if config.display_preamble() && terminal::supports_notify_send() && !config.skip_notify() {
             print_warning("Due to a design issue with notify-send it could be that topgrade hangs when it's finished.
 If this is the case on your system add the --skip-notify flag to the topgrade command or set skip_notify = true in the config file.
 If you don't want this message to appear any longer set display_preamble = false in the config file.

src/terminal.rs

@@ -345,3 +345,8 @@ pub fn notify_desktop<P: AsRef<str>>(message: P, timeout: Option<Duration>) {
 pub fn display_time(display_time: bool) {
     TERMINAL.lock().unwrap().display_time(display_time);
 }
+
+#[cfg(target_os = "linux")]
+pub fn supports_notify_send() -> bool {
+    TERMINAL.lock().unwrap().notify_send.is_some()
+}