perf tools: Makefile: Clean up `python/perf.so' rule

perf tools: Makefile: Clean up `python/perf.so' rule

There is no need for a subshell or an explicit `export';
as per the POSIX Shell Command Language specification:

  http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_09_01
  http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_10_02

It is only necessary to include the environment variable
assignment just before the command to be run.

Also, it is better to use single-quotes, because GNU make
might expand `$(BASIC_CFLAGS)' into something that the shell
could interpret within double-quotes.

Acked-by: Raghavendra D Prabhu <rprabhu@wnohang.net>
Link: http://lkml.kernel.org/n/tip-58n38o02ocuzrm9qh096hsf5@git.kernel.org
Signed-off-by: Michael Witten <mfwitten@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>