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 <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Vineet Gupta <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/config/feature-checks/Makefile

···
 	$(BUILD)
 
 test-pthread-attr-setaffinity-np.bin:
-	$(BUILD) -Werror -lpthread
+	$(BUILD) -D_GNU_SOURCE -Werror -lpthread
 
 test-stackprotector-all.bin:
 	$(BUILD) -Werror -fstack-protector-all