message: [build] remove autoconf config.h
diff: #ifndef OTBR_CONFIG_H_
  #define OTBR_CONFIG_H_
  -#if HAVE_CONFIG_H
  -#include "autoconf-config.h"
  -#elif defined(OTBR_CONFIG_FILE)
  +#if defined(OTBR_CONFIG_FILE)
  #include OTBR_CONFIG_FILE
  #endif
  