The 'tools/perf/test/make' makefile has in its default, 'all' target
builds that will pollute the source code directory, i.e. that will not
use O= variable.

The 'build-test' should be run as often as possible, preferrably after
each non strictly non-code commit, so speed it up by selecting just
the O= targets.

Furthermore it tests both the Makefile.perf file, that is normally
driven by the main Makefile, and the Makefile, reduce the time in half
by having just MK=Makefile, the most usual, tested by 'build-test'.

Please run:

  make -C tools/perf -f tests/make

from time to time for testing also the in-place build tests.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-jrt9utscsiqkmjy3ccufostd@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>