Fix examples to build cleanly if PICO_DEFAULT_LED_PIN isn't defined

blink/blink.c

@@ -7,6 +7,9 @@
 #include "pico/stdlib.h"
 
 int main() {
+#ifndef PICO_DEFAULT_LED_PIN
+#warning blink 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);
@@ -16,4 +19,5 @@ int main() {
         gpio_put(LED_PIN, 0);
         sleep_ms(250);
     }
+#endif
 }