perf callchains: Disable unwind libraries when libelf isn't found

I.e. do the same as when NO_LIBELF is explicitely passed in the 'make'
command line, fixing this:

  Auto-detecting system features:
  ...                         dwarf: [ OFF ]
  ...                         glibc: [ on  ]
  ...                          gtk2: [ OFF ]
  ...                      libaudit: [ OFF ]
  ...                        libbfd: [ OFF ]
  ...                        libelf: [ OFF ]
  ...                     libunwind: [ OFF ]
  ...            libdw-dwarf-unwind: [ OFF ]
  ...     DWARF post unwind library: libdw
<SNIP>
    CC       /tmp/build/perf/util/symbol-minimal.o
    CC       /tmp/build/perf/util/unwind-libdw.o
  arch/x86/util/unwind-libdw.c:1:30: fatal error: elfutils/libdwfl.h: No such file or directory
  compilation terminated.
    CC       /tmp/build/perf/tests/keep-tracking.o
  util/unwind-libdw.c:2:28: fatal error: elfutils/libdw.h: No such file or directory
  compilation terminated.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-e39j1yxanltjx4t0msse63ax@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>