message: Use printf portably.
  PRId64 is defined to the right printf format string to print a int64_t on the current system.
diff: +#include <inttypes.h>
  #include <stdio.h>
  #include "../wasmer.h"
  #include <assert.h>
  @@ -242,7 +243,7 @@ int main()
  wasmer_result_t call_result = wasmer_export_func_call(exported_function, inputs, inputs_arity, outputs, outputs_arity);
  - printf("Result: %lld\n", outputs[0].value.I64);
  + printf("Result: %" PRId64 "\n", outputs[0].value.I64);
  assert(outputs[0].value.I64 == 7);
  assert(call_result == WASMER_OK);
  