perf tools: Let O= makes handle relative paths

perf tools: Let O= makes handle relative paths

When I did a compile of perf using a relative path for the output
directory, the build failed when it tried to compile libtraceevent. This
is because it continues to use the same relative path when the new
working directory is in a different path.

    SUBDIR ../lib/traceevent/
/bin/sh: line 0: cd: ../../../nobackup/perf/: No such file or directory
Makefile:74: *** output directory "../../../nobackup/perf/" does not exist.  Stop.
make: *** [../../../nobackup/perf///libtraceevent.a] Error 2

Make the path used an absolute path when building perf with O=.

Boris:

Teach Makefile to check whether the supplied O= directory exists and
bail out if not. Reportedly, kernel dudes are idiots and need to be
guarded so as not to shoot themselves in the foot when playing in the
sandbox.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20120815163923.GD15989@aftab.osrc.amd.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>