Added warning message when not including build_config.h
Added warning message when not including build_config.h

Before:
/media/ssd/nacl/native_client/src/trusted/debug_stub/target.cc: build_config
  line 29: ['#if NACL_WINDOWS'] \
  line 183: ['    if ((NACL_ARCH(NACL_BUILD_ARCH) == NACL_x86 &&'] \
  line 185: ['        (NACL_ARCH(NACL_BUILD_ARCH) == NACL_arm &&'] \
  line 451: ['  if (NACL_ARCH(NACL_BUILD_ARCH) == NACL_x86 && NACL_BUILD_SUBARCH == 64 &&']

After:
/media/ssd/nacl/native_client/src/trusted/debug_stub/target.cc: build_config
  line 29: ['#if NACL_WINDOWS'] must include "native_client/src/include/build_config.h" \
  line 183: ['    if ((NACL_ARCH(NACL_BUILD_ARCH) == NACL_x86 &&'] must include "native_client/src/include/build_config.h" \
  line 185: ['        (NACL_ARCH(NACL_BUILD_ARCH) == NACL_arm &&'] must include "native_client/src/include/build_config.h" \
  line 451: ['  if (NACL_ARCH(NACL_BUILD_ARCH) == NACL_x86 && NACL_BUILD_SUBARCH == 64 &&'] must include "native_client/src/include/build_config.h"

BUG=None

Review URL: https://codereview.chromium.org/1214343008

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client