perf tools: Factor features display code

Currently the we display all detected features/libraries by following
rules:

 - if one of the features is missing
 - if it's build from clean tree

This patch changes changes this behavior in several ways.

- We no longer display all detected features, only detected libraries
  are displayed by default:

  $ make
    BUILD:   Doing 'make -j4' parallel build

  Auto-detecting system features:
  ...                         dwarf: [ on  ]
  ...                         glibc: [ on  ]
  ...                          gtk2: [ on  ]
  ...                      libaudit: [ on  ]
  ...                        libbfd: [ on  ]
  ...                        libelf: [ on  ]
  ...                       libnuma: [ on  ]
  ...                       libperl: [ on  ]
  ...                     libpython: [ on  ]
  ...                      libslang: [ on  ]
  ...                     libunwind: [ on  ]

  The assumption is, that above libraries are the most interesting part
of the detection, while we don't care much about detection of on-exit
support.

- If all above libraries are detected, the default is not shown on
  subsequent builds.

- If one of the above libraries is missing, the detection output is
  forced.

- The features status is stored in PERF-FEATURES file and the detection
  output is forced in case the there's difference between the file
  contents and currently detected features.

- If you want to see all detected features, you can use VF=1 make
  variable, that forces the detected features output.

  $ make VF=1
    BUILD:   Doing 'make -j4' parallel build

  Auto-detecting system features:
  ...                         dwarf: [ on  ]
  ...                         glibc: [ on  ]
  ...                          gtk2: [ on  ]
  ...                      libaudit: [ on  ]
  ...                        libbfd: [ on  ]
  ...                        libelf: [ on  ]
  ...                       libnuma: [ on  ]
  ...                       libperl: [ on  ]
  ...                     libpython: [ on  ]
  ...                      libslang: [ on  ]
  ...                     libunwind: [ on  ]
  ...                     backtrace: [ on  ]
  ...                fortify-source: [ on  ]
  ...                  gtk2-infobar: [ on  ]
  ...             libelf-getphdrnum: [ on  ]
  ...                   libelf-mmap: [ on  ]
  ...             libpython-version: [ on  ]
  ...                       on-exit: [ on  ]
  ...            stackprotector-all: [ on  ]
  ...                       timerfd: [ on  ]
  ...         libunwind-debug-frame: [ OFF ]
  ...                        bionic: [ OFF ]

Signed-off-by: Jiri Olsa <[email protected]>
Cc: Corey Ashford <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jean Pihet <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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