perf tools: Correct Makefile.include

It looks at O= and adjusts the $(OUTPUT) variable based on what the
output directory will be. However, when O is defined but empty, it
wrongly becomes the user's $HOME dir which is not what we want. So check
it is not empty before working with it further.
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1361374353-30385-4-git-send-email-bp@alien8.deSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/scripts/Makefile.include

+ifneq ($(O),)
 ifeq ($(origin O), command line)
 	dummy := $(if $(shell test -d $(O) || echo $(O)),$(error O=$(O) does not exist),)
 	ABSOLUTE_O := $(shell cd $(O) ; pwd)
@@ -7,9 +8,10 @@ ifeq ($(objtree),)
 	objtree := $(O)
 endif
 endif
+endif
 
-ifneq ($(OUTPUT),)
 # check that the output directory actually exists
+ifneq ($(OUTPUT),)
 OUTDIR := $(shell cd $(OUTPUT) && /bin/pwd)
 $(if $(OUTDIR),, $(error output directory "$(OUTPUT)" does not exist))
 endif