perf tools: Use shell function for perl cflags retrieval

perf tools: Use shell function for perl cflags retrieval

Using the shell function for perl CFLAGS retrieval instead of back
quotes (``). Both execute shell with the command, but the latter is more
explicit and seems to be the preferred way.

Also we don't have any other use of the back quotes in perf Makefiles.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20171108102739.30338-2-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>