As noted by @asomers in rS300906 (`cddl/usr.sbin/zfsd/tests/Makefile`
specifically), googletest requires libpthread in order to function, if
`-DGTEST_HAS_PTHREAD` is defined.
While this is true, the code was not being linked against libpthread,
resulting in libgtest being broken, and thus all tests reliant on the library
broken at runtime.
Suggested by: @jbeich