BUILD_BPF_SKEL error message

[Arnaldo Carvalho de Melo]

I switched to Fedora 34 Silverblue, and while installing the needed
packages to build perf, using BUILD_BPF_SKEL=1 I noticed this:

Makefile.config:637: *** Error: clang too old. Please install recent clang.  Stop.

The problem is not that clang is too old, its that it is not installed
:-)

Perhaps a slight improvement to state that, something like:

Makefile.config:637: *** Error: clang too old/not installed. Please install recent clang to build with BPF_SKEL.  Stop.

As for a moment I thought clang now was _required_ to build perf, which
isn't the case, it was just me using an alias to build perf:

alias m='make -k BUILD_BPF_SKEL=1 PYTHON=python3 O=/tmp/build/perf -C tools/perf install-bin && perf test python'

Regards,

- Arnaldo