-lrt is not needed in OpenBSD.

clock_gettime is in libc in OpenBSD. [1]

[1] http://www.openbsd.org/cgi-bin/cvsweb/ports/devel/ruby-prof/patches/patch-ext_ruby_prof_extconf_rb?rev=1.1
Merge pull request #119 from edogawaconan/openbsd

-lrt is not needed in OpenBSD.