Fix examples to build cleanly if PICO_DEFAULT_LED_PIN isn't defined

system/hello_double_tap/hello_double_tap.c

@@ -13,6 +13,9 @@
 // `pico_bootsel_via_double_reset` library!
 
 int main() {
+#ifndef PICO_DEFAULT_LED_PIN
+#warning system/hello_double_tap example requires a board with a regular LED
+#else
     const uint LED_PIN = PICO_DEFAULT_LED_PIN;
     gpio_init(LED_PIN);
     gpio_set_dir(LED_PIN, GPIO_OUT);
@@ -22,4 +25,5 @@ int main() {
         gpio_put(LED_PIN, 0);
         sleep_ms(250);
     }
+#endif
 }