perf tools: Do not use _FORTIFY_SOURCE when DEBUG=1 is specified

As:

make DEBUG=1 -C tools/perf

disables optimizations and _FORTIFY_SOURCE in recent distros requires
optimizations to be enabled, seen on a Fedora 17 system:

[acme@Fedora17 linux]$ make DEBUG=1 O=/home/acme/git/build/perf/ -C
tools/perf install
In file included from /usr/include/sys/types.h:26:0,
                 from /usr/include/libelf.h:53,
                 from /usr/include/gelf.h:53,
                 from /usr/include/elfutils/libdw.h:53,
                 from <stdin>:2:
/usr/include/features.h:314:4: error: #warning _FORTIFY_SOURCE requires
compiling with optimization (-O) [-Werror=cpp

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-4ccyiebqju4uatm31ky7725b@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile

@@ -80,7 +80,7 @@ ifeq ("$(origin DEBUG)", "command line")
   PERF_DEBUG = $(DEBUG)
 endif
 ifndef PERF_DEBUG
-  CFLAGS_OPTIMIZE = -O6
+  CFLAGS_OPTIMIZE = -O6 -D_FORTIFY_SOURCE=2
 endif
 
 ifdef PARSER_DEBUG
@@ -89,7 +89,7 @@ ifdef PARSER_DEBUG
 	PARSER_DEBUG_CFLAGS := -DPARSER_DEBUG
 endif
 
-CFLAGS = -fno-omit-frame-pointer -ggdb3 -Wall -Wextra -std=gnu99 $(CFLAGS_WERROR) $(CFLAGS_OPTIMIZE) -D_FORTIFY_SOURCE=2 $(EXTRA_WARNINGS) $(EXTRA_CFLAGS) $(PARSER_DEBUG_CFLAGS)
+CFLAGS = -fno-omit-frame-pointer -ggdb3 -Wall -Wextra -std=gnu99 $(CFLAGS_WERROR) $(CFLAGS_OPTIMIZE) $(EXTRA_WARNINGS) $(EXTRA_CFLAGS) $(PARSER_DEBUG_CFLAGS)
 EXTLIBS = -lpthread -lrt -lelf -lm
 ALL_CFLAGS = $(CFLAGS) -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE
 ALL_LDFLAGS = $(LDFLAGS)