clang is required to build perf with BUILD_BPF_SKEL=1. Improve the
error message to highlight that:

  1) clang could be either missing or too old;
  2) clang is only required with BUILD_BPF_SKEL=1.
Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>