perf python: More portable way to make CFLAGS work with clang

The existing code that tries to make CFLAGS compatible with clang
doesn't work with Python 3.

Instead of trying to touch _sysconfigdata.build_time_vars directly,
change the dictionary returned by disutils.sysconfig.get_config_vars().
This works on both Python 2 and Python 3.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20181005204058.7966-3-ehabkost@redhat.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>