perf tools: Define _GNU_SOURCE on pthread_attr_setaffinity_np feature check

The man page for pthread_attr_set_affinity_np states that _GNU_SOURCE
must be defined before pthread.h is included in order to get the proper
function declaration.  Define this in the Makefile.

Without this defined, the feature check fails on a Fedora system with
gcc5 and then the perf build later fails with conflicting prototypes for
the function.

Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Link: http://lkml.kernel.org/r/20150211162404.GA15522@hansolo.redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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