Before this patch libbpf always do feature check even when cleaning.

For example:

  $ cd kernel/tools/lib/bpf
  $ make

  Auto-detecting system features:
  ...                        libelf: [ on  ]
  ...                           bpf: [ on  ]

    CC       libbpf.o
    CC       bpf.o
    LD       libbpf-in.o
    LINK     libbpf.a
    LINK     libbpf.so
  $ make clean
    CLEAN    libbpf
    CLEAN    core-gen
  $ make clean

  Auto-detecting system features:
  ...                        libelf: [ on  ]
  ...                           bpf: [ on  ]

    CLEAN    libbpf
    CLEAN    core-gen
  $

Although the first 'make clean' doesn't show feature check result, it
still does the check. No output because check result is similar to
FEATURE-DUMP.libbpf.

This patch uses same method as perf to turn off feature checking when
'make clean'.
Reported-and-Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1448372181-151723-3-git-send-email-wangnan0@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>