tools/perf/build: Fix non-canonical directory names in O=

This was a long-standing bug, relative pathnames like O=dir did not fully
work in the build system:

    $ make O=localdir clean

    SUBDIR Documentation
    ../../scripts/Makefile.include:3: *** O=localdir does not exist.  Stop.
    make[1]: *** [clean] Error 2
    make: *** [clean] Error 2

Fix this by canonizing the directory before passing it to Makefile.perf.

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-hchMp1hozn9tqgswWcooxcru@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/perf/Makefile

@@ -30,12 +30,19 @@ ifeq ($(JOBS),)
   endif
 endif
 
+#
+# Only pass canonical directory names as the output directory:
+#
+ifneq ($(O),)
+  FULL_O := $(shell readlink -f $(O))
+endif
+
 define print_msg
   @printf '    BUILD: Doing '\''make \033[33m-j'$(JOBS)'\033[m'\'' parallel build\n'
 endef
 
 define make
-  @$(MAKE) -f Makefile.perf --no-print-directory -j$(JOBS) O=$(O) $@
+  @$(MAKE) -f Makefile.perf --no-print-directory -j$(JOBS) O=$(FULL_O) $@
 endef
 
 #