perf build: Gracefully fail the build if BUILD_BPF_SKEL=1 is specified and clang isn't available

Build BPF skels require having a compiler able to generate BPF bytecode,
and so far this is only possible with clang, so check for its
availability and fail the build when the user explicitely ask for BPF
skels to be built.

Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Song Liu <songliubraving@meta.com>
Yang: Yang Jihong <yangjihong1@huawei.com>,
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>