perf build: Do not fail on missing Build file

Allow nesting into directories without Build file. Currently we force
include of the Build file, which fails the build when the Build file is
missing.

We already support empty *-in.o' objects if there's nothing in the
directory to be compiled, so we can just use it for missing Build file
cases.

Also adding this case under tests.

Reported-by: Rabin Vincent <rabin.vincent@axis.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Rabin Vincent <rabin.vincent@axis.com>
Link: http://lkml.kernel.org/r/1432914178-24086-1-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>