perf tools: Fix lib/traceevent build dir with OUTPUT variable set

perf tools: Fix lib/traceevent build dir with OUTPUT variable set

With the OUTPUT variable set the libtraceevent.a file is wrongly built
in the source directory:

 + make -d OUTPUT=/.../.build/perf-user/ DESTDIR=/.../.install/perf-user/
 ...
     Considering target file `../lib/traceevent//libtraceevent.a'.
      File `../lib/traceevent//libtraceevent.a' does not exist.
      Finished prerequisites of target file `../lib/traceevent//libtraceevent.a'.
     Must remake target `../lib/traceevent//libtraceevent.a'.
 Invoking recipe from Makefile:837 to update target `../lib/traceevent//libtraceevent.a'.
 Putting child 0x703850 (../lib/traceevent//libtraceevent.a) PID 8365 on the chain.
 Live child 0x703850 (../lib/traceevent//libtraceevent.a) PID 8365
     SUBDIR ../lib/traceevent/

 $ git clean -nxd
 Would remove tools/lib/traceevent/.event-parse.d
 Would remove tools/lib/traceevent/.parse-filter.d
 Would remove tools/lib/traceevent/.parse-utils.d
 Would remove tools/lib/traceevent/.trace-seq.d
 Would remove tools/lib/traceevent/event-parse.o
 Would remove tools/lib/traceevent/libtraceevent.a
 Would remove tools/lib/traceevent/parse-filter.o
 Would remove tools/lib/traceevent/parse-utils.o
 Would remove tools/lib/traceevent/trace-seq.o

This patch fixes this.

Note: Though this should already work with O=$outputdir we better use
the OUTPUT variable directly.

Signed-off-by: Robert Richter <robert.richter@amd.com>
Cc: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/1344361396-7237-3-git-send-email-robert.richter@amd.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>