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.de


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>