perf tools: Fix make PYTHON override

perf tools: Fix make PYTHON override

Thomas reported that make PYTHON=python2 is not work on some systems.  I
can reproduce it on my ArchLinux box too.

This is because it's overridden by config/feature-checks/Makefile
regardless of PYTHON setting.  I guess it's a bug slipped into during
the feature checking change.

Actually, we don't need to check python-config in the feature-checks.
We can just pass appropriate FEATURE_CHECK_*FLAGS.

Reported-by: Thomas Ilsche <thomas.ilsche@tu-dresden.de>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Thomas Ilsche <thomas.ilsche@tu-dresden.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Ilsche <thomas.ilsche@tu-dresden.de>
Link: http://lkml.kernel.org/r/1406617040-26909-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>