perf tools: Fix output directory of Documentation/

The OUTPUT directory is wrongly determind leading to:

 make[3]: *** No rule to make target `.../.build/perf/PERF-VERSION-FILE'.  Stop.

Fixing this by using the generic approach in script/Makefile.include.
Signed-off-by: NRobert Richter <robert.richter@calxeda.com>
Link: http://lkml.kernel.org/r/1367865614-30876-1-git-send-email-rric@kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/Makefile

+include ../../scripts/Makefile.include
 include ../config/utilities.mak
 
-OUTPUT := ./
-ifeq ("$(origin O)", "command line")
-  ifneq ($(O),)
-	OUTPUT := $(O)/
-  endif
-endif
-
 MAN1_TXT= \
 	$(filter-out $(addsuffix .txt, $(ARTICLES) $(SP_ARTICLES)), \
 		$(wildcard perf-*.txt)) \
@@ -277,7 +271,7 @@ $(MAN_HTML): $(OUTPUT)%.html : %.txt
 
 $(OUTPUT)%.1 $(OUTPUT)%.5 $(OUTPUT)%.7 : $(OUTPUT)%.xml
 	$(QUIET_XMLTO)$(RM) $@ && \
-	$(XMLTO) -o $(OUTPUT) -m $(MANPAGE_XSL) $(XMLTO_EXTRA) man $<
+	$(XMLTO) -o $(OUTPUT). -m $(MANPAGE_XSL) $(XMLTO_EXTRA) man $<
 
 $(OUTPUT)%.xml : %.txt
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \