Clear compile errors if BT needed but not on (#1371)

Fixes #1370

Adds a simple helper assertion to tell the user how to enable BT if it's
not enabled, instead of some odd compilation warnings about undefined
functions.

cores/rp2040/_needsbt.h

+// Simple helper header to ensure pico libs support ?BT
+
+#ifndef ENABLE_CLASSIC
+#define ENABLE_CLASSIC 0
+#endif
+
+static_assert(ENABLE_CLASSIC, "This library needs Bluetooth enabled.  Use the 'Tools->IP/Bluetooth Stack' menu in the IDE to enable it.");

libraries/BTstackLib/src/BTstackLib.h

 /**
     Arduino Wrapper for BTstack
 */
+
+#include <_needsbt.h>
+
 #ifndef __ARDUINO_BTSTACK_H
 #define __ARDUINO_BTSTACK_H
 
@@ -198,4 +201,4 @@ extern "C" {
 }
 #endif
 
-#endif // __ARDUINO_BTSTACK_H
\ No newline at end of file
+#endif // __ARDUINO_BTSTACK_H

libraries/HID_Bluetooth/src/PicoBluetoothBLEHID.h

@@ -21,6 +21,7 @@
 
 #pragma once
 
+#include <_needsbt.h>
 #include <Arduino.h>
 #include <functional>
 #include <pico/cyw43_arch.h>

libraries/HID_Bluetooth/src/PicoBluetoothHID.h

@@ -21,6 +21,7 @@
 
 #pragma once
 
+#include <_needsbt.h>
 #include <Arduino.h>
 #include <functional>
 #include <pico/cyw43_arch.h>

libraries/SerialBT/src/SerialBT.h

@@ -20,6 +20,7 @@
 
 #pragma once
 
+#include <_needsbt.h>
 #include <Arduino.h>
 #include <api/HardwareSerial.h>
 #include <stdarg.h>