perf doc: Allow producing documentation in a specified output directory

Currently we can put the object files in a different directory by using
'O=' comand line argument.

However the generated documentation files don't honor this directive,

This patch fixes that. It's been tested for man target but the others
seems currently broken so no tests have been done on them so far.

Link: http://lkml.kernel.org/r/1328541443-18003-1-git-send-email-fbuihuu@gmail.com
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>