perf tools: check if -DFORTIFY_SOURCE=2 is allowed

It seems gcc (4.7.2) defines _FORTIFY_SOURCE internally and becomes
confused when it sees another definition in flags.

For me, build failed like this:

CHK glibc
Makefile:548: *** No gnu/libc-version.h found, please install glibc-dev[el]/glibc-static. Stop.

and only with V=1 it printed:

<command-line>:0:0: error: "_FORTIFY_SOURCE" redefined [-Werror]
<stdin>:1:0: note: this is the location of the previous definition

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1361113416-8662-1-git-send-email-marcin.slusarz@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile

@@ -95,7 +95,7 @@ ifeq ("$(origin DEBUG)", "command line")
   PERF_DEBUG = $(DEBUG)
 endif
 ifndef PERF_DEBUG
-  CFLAGS_OPTIMIZE = -O6 -D_FORTIFY_SOURCE=2
+  CFLAGS_OPTIMIZE = -O6
 endif
 
 ifdef PARSER_DEBUG
@@ -180,6 +180,12 @@ ifeq ($(call try-cc,$(SOURCE_HELLO),$(CFLAGS) -Werror -Wvolatile-register-var,-W
        CFLAGS := $(CFLAGS) -Wvolatile-register-var
 endif
 
+ifndef PERF_DEBUG
+	ifeq ($(call try-cc,$(SOURCE_HELLO),$(CFLAGS) -D_FORTIFY_SOURCE=2,-D_FORTIFY_SOURCE=2),y)
+		CFLAGS := $(CFLAGS) -D_FORTIFY_SOURCE=2
+	endif
+endif
+
 ### --- END CONFIGURATION SECTION ---
 
 ifeq ($(srctree),)