Makefile parallel build race condition

[Adi Ratiu]

(see attached patch)

Fix a race condition causing parallel builds to sometimes fail.

Both probe_i8k_calls_time and i8kctl contain a main() function, though
i8kctl's is omitted when built with -DLIB so they can link without a
conflict. i8kctl is also a standalone exe so it is built twice, with
and without -DLIB. When building in parallel you can get into a
situation where the object file created by the probe_i8k_calls_time
target gets overwritten with the one from i8kctl and bad things happen.
Nothing actually uses the i8kctl.o that has main() so we can just not
build it.

Initially reported at: https://bugs.gentoo.org/show_bug.cgi?id=565466

[vitorafsr (vitorafsr)]

Fixed on revision 51.