don't build / link-in a main() by default

allow lib users to implement main() by only compiling the default main()
if TEENSY_CORE_MAIN is defined

src/core/main.cpp

@@ -28,6 +28,7 @@
  * SOFTWARE.
  */
 
+#ifdef TEENSY_CORE_MAIN
 #include <core/Arduino.h>
 
 extern "C" int main(void)
@@ -55,4 +56,4 @@ extern "C" int main(void)
 	}
 #endif
 }
-
+#endif /* TEENSY_CORE_MAIN */