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While building on a wide range of distros and clang versions it was
noticed that at least version 12.0.1 (noticed on Alpine 3.15 with
"Alpine clang version 12.0.1") is needed to not fail with BTF generation
errors such as:

Debian:10

  Debian clang version 11.0.1-2~deb10u1:

    CLANG   /tmp/build/perf/util/bpf_skel/.tmp/sample_filter.bpf.o
  <SNIP>
    GENSKEL /tmp/build/perf/util/bpf_skel/sample_filter.skel.h
  libbpf: failed to find BTF for extern 'bpf_cast_to_kern_ctx' [21] section: -2
  Error: failed to open BPF object file: No such file or directory
  make[2]: *** [Makefile.perf:1121: /tmp/build/perf/util/bpf_skel/sample_filter.skel.h] Error 254
  make[2]: *** Deleting file '/tmp/build/perf/util/bpf_skel/sample_filter.skel.h'

Amazon Linux 2:

  clang version 11.1.0 (Amazon Linux 2 11.1.0-1.amzn2.0.2)

    GENSKEL /tmp/build/perf/util/bpf_skel/sample_filter.skel.h
  libbpf: elf: skipping unrecognized data section(18) .eh_frame
  libbpf: elf: skipping relo section(19) .rel.eh_frame for section(18) .eh_frame
  libbpf: failed to find BTF for extern 'bpf_cast_to_kern_ctx' [21] section: -2
  Error: failed to open BPF object file: No such file or directory
  make[2]: *** [/tmp/build/perf/util/bpf_skel/sample_filter.skel.h] Error 254
  make[2]: *** Deleting file `/tmp/build/perf/util/bpf_skel/sample_filter.skel.h'

Ubuntu 20.04:

  clang version 10.0.0-4ubuntu1

    CLANG   /tmp/build/perf/util/bpf_skel/.tmp/augmented_raw_syscalls.bpf.o
    GENSKEL /tmp/build/perf/util/bpf_skel/bench_uprobe.skel.h
    GENSKEL /tmp/build/perf/util/bpf_skel/bperf_leader.skel.h
  libbpf: sec '.reluprobe': corrupted symbol #27 pointing to invalid section #65522 for relo #0
    GENSKEL /tmp/build/perf/util/bpf_skel/bperf_follower.skel.h
  Error: failed to open BPF object file: BPF object format invalid
  make[2]: *** [Makefile.perf:1121: /tmp/build/perf/util/bpf_skel/bench_uprobe.skel.h] Error 95
  make[2]: *** Deleting file '/tmp/build/perf/util/bpf_skel/bench_uprobe.skel.h'

So check if the version is at least 12.0.1 otherwise disable building
BPF skels and provide a message about it, continuing the build.

The message, when running on amazonlinux:2:

  Makefile.config:698: Warning: Disabled BPF skeletons as reliable BTF generation needs at least clang version 12.0.1
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/ZTvGx/Ou6BVnYBqi@kernel.org

Signed-off-by: Namhyung Kim <namhyung@kernel.org>