[test] use CLOCK_PROF for NetBSD build

This time it builds fine: http://robotpkg.openrobots.org/rbulk/robotpkg-wip/wip/libnabo
It might need some test though. The same patch apply to `libpointmatcher`.

*cf.* http://www.freebsd.org/cgi/man.cgi?query=clock_gettime