perf tools: Warn the user about how to enable libunwind support

perf tools: Warn the user about how to enable libunwind support

When one has libunwind installed somewhere the perf tools build process
doesn't expects it to be, this happens:

  [acme@ssdandy linux]$ make O=/tmp/build/perf -C tools/perf/ install-bin
  make: Entering directory `/home/acme/git/linux/tools/perf'
    BUILD:   Doing 'make -j8' parallel build
  config/Makefile:312: No libunwind found, disabling post unwind support.  Please install libunwind-dev[el] >= 1.1

  Auto-detecting system features:
  <SNIP>
  ...                     libunwind: [ OFF ]

Change the message so that it tells how to use a non-standard libunwind
install directory:

  config/Makefile:312: No libunwind found, disabling post unwind support.  Please install libunwind-dev[el] >= 1.1 and/or set LIBUNWIND_DIR

  [acme@ssdandy linux]$ make LIBUNWIND_DIR=/opt/libunwind-git/ O=/tmp/build/perf -C tools/perf/ install-bin
  make: Entering directory `/home/acme/git/linux/tools/perf'
    BUILD:   Doing 'make -j8' parallel build

  Auto-detecting system features:
  <SNIP>
  ...                     libunwind: [ on  ]

Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.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-huoxnou7sw85lm58k3pi1xhw@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>