netdev - How to detect libbfd when building tools/bpf?

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Date:   Sun, 22 Apr 2018 15:51:40 +0800
From:   "Wang Sheng-Hui" <shhuiw@...mail.com>
To:     "ast" <ast@...nel.org>,
        "daniel" <daniel@...earbox.net>,
        "kstewart" <kstewart@...uxfoundation.org>,
        "gregkh" <gregkh@...uxfoundation.org>,
        "tglx" <tglx@...utronix.de>,
        "pombredanne" <pombredanne@...b.com>
Cc:     "netdev" <netdev@...r.kernel.org>
Subject: How to detect libbfd when building tools/bpf?

Sorry to trouble you !

I run debian and installed binutils-dev beforehand.
Then I copied tools/build/feature/test-libbfd.c to t.c and run:
------------------------------------------------------------------------
root@...yfintech:~# cat t.c 
#include <bfd.h>

extern int printf(const char *format, ...);

int main(void)
{
	char symbol[4096] = "FieldName__9ClassNameFd";
	char *tmp;

	tmp = bfd_demangle(0, symbol, 0);

	printf("demangled symbol: {%s}\n", tmp);

	return 0;
}
root@...yfintech:~# gcc t.c -lbfd
root@...yfintech:~# ./a.out 
demangled symbol: {ClassName::FieldName}


I thought libbfd can be reached from above. 
But when I built tools/bpf, libbfd cannot be detected: 
-------------------------------------------------------------------------
/linux# make O=../buildkernel/ tools/bpf
make[1]: Entering directory '/root/buildkernel'
  DESCEND  bpf

Auto-detecting system features:
...                        libbfd: [ OFF ]
...        disassembler-four-args: [ on  ]

  DESCEND  bpftool

Auto-detecting system features:
...                        libbfd: [ OFF ]
...        disassembler-four-args: [ on  ]

make[1]: Leaving directory '/root/buildkernel'


I wonder how should I do to get libbfd auto detected when  building bpf tool?


Regards,
shenghui