Instead of doing an unconditional mkdir, use a dummy Makefile variable
to check if the directory is there and if not, create it.

This is better than what we had and will help with other python bindings
that are in development, like one involved with python backtraces.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-iis6us2nocw3y4uuoon9osd7@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>