tools build feature: Don't set feature-libcap=1 if libcap-devel isn't available

libcap isn't tested in the tools/build/feature/test-all.c fast path
feature detection process, so don't set it as available if test-all
manages to build.

There are other users of this feature detection mechanism, and they
explicitely ask for libcap to be tested, so are not affected by this
patch, for instance, with this patch in place:

  $ make -C tools/bpf/bpftool/ clean
  <SNIP>
  make: Leaving directory '/home/acme/git/perf-tools-next/tools/bpf/bpftool'
  ⬢ [acme@toolbox perf-tools-next]$ make -C tools/bpf/bpftool/
  make: Entering directory '/home/acme/git/perf-tools-next/tools/bpf/bpftool'

  Auto-detecting system features:
  ...                         clang-bpf-co-re: [ on  ]
  ...                                    llvm: [ on  ]
  ...                                  libcap: [ on  ]
  ...                                  libbfd: [ on  ]
  ...                             libelf-zstd: [ on  ]
  <SNIP>
    LINK    bpftool
  make: Leaving directory '/home/acme/git/perf-tools-next/tools/bpf/bpftool'
  $
  $ sudo rpm -e libcap-devel
  $ make -C tools/bpf/bpftool/
  <SNIP>
  make: Entering directory '/home/acme/git/perf-tools-next/tools/bpf/bpftool'

  Auto-detecting system features:
  ...                         clang-bpf-co-re: [ on  ]
  ...                                    llvm: [ on  ]
  ...                                  libcap: [ OFF ]
  ...                                  libbfd: [ on  ]
  ...                             libelf-zstd: [ on  ]

  $

Reviewed-by: Ian Rogers <irogers@google.com>
Acked-by: Quentin Monnet <qmo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/lkml/20241211224509.797827-3-acme@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>