If either NO_LIBBPF=1 is passed, explicitely disabling it or if libbpf
is not available due to some missing dependency, skip its tests, telling
the user the feature isn't available.

  # perf test
  <SNIP>
  40: LLVM search and compile                                         : Skip (not compiled in)
  41: Session topology                                                : Ok
  42: BPF filter                                                      : Skip (not compiled in)
  <SNIP>

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>