perf build: Improve error message for old/missing clang

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>

tools/perf/Makefile.config

@@ -635,7 +635,7 @@ endif
 ifdef BUILD_BPF_SKEL
   $(call feature_check,clang-bpf-co-re)
   ifeq ($(feature-clang-bpf-co-re), 0)
-    dummy := $(error Error: clang too old. Please install recent clang)
+    dummy := $(error Error: clang too old/not installed. Please install recent clang to build with BUILD_BPF_SKEL)
   endif
   $(call detected,CONFIG_PERF_BPF_SKEL)
   CFLAGS += -DHAVE_BPF_SKEL