perf tools: Pass PYTHON config to feature detection

perf tools: Pass PYTHON config to feature detection

( This is a rebased version of https://lkml.org/lkml/2017/2/7/662 )

Python's CC and link Makefile variables were not passed to feature
detection, causing feature detection to use system's Python rather than
PYTHON_CONFIG's one. This created a mismatch between the detected Python
support and the one actually used by perf when PYTHON_CONFIG is
specified.

Fix it by moving Python's variable initialization to before feature
detection and pass FLAGS_PYTHON_EMBED to Python's feature detection's
build target.

Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Simon Que <sque@chromium.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20170412064919.92449-2-davidcc@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>