perf tools: Warn about missing libelf

When perf detects no libelf during the build, it'll use internal mini
elf parser instead of libelf.  But as it only supports minimal
functionalities, it also disables support to 'probe' builtin command.

Currently it didn't warned to user.  Fix it.

$ sudo apt-get remove libelf-dev
$ make
    CHK -fstack-protector-all
    CHK -Wstack-protector
    CHK -Wvolatile-register-var
    CHK bionic
    CHK libelf
    CHK glibc
Makefile:491: No libelf found, disables 'probe' tool, please install elfutils-libelf-devel/libelf-dev
    CHK libunwind
    CHK libaudit

$ make NO_LIBELF=1
    CHK -fstack-protector-all
    CHK -Wstack-protector
    CHK -Wvolatile-register-var
    CHK bionic
    CHK libaudit

Reported-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-8ww8zc4hhpxabfskxs3u5ede@git.kernel.org


[ committer note: The package needed is elfutils-libelf-devel, not elfutils-devel ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>