Fix test failure on Debian GNU/kFreeBSD (and probably GNU/Hurd)

This pair of commits fixes a compiler warning and a test failure on at least Debian GNU/kFreeBSD (which uses the FreeBSD kernel but the GNU libc):

  https://buildd.debian.org/status/fetch.php?pkg=haskell-unix-time&arch=kfreebsd-amd64&ver=0.1.2-3&stamp=1370013165

The GNU/Hurd failure looks similar, and I expect this to fix it too:

  https://buildd.debian.org/status/fetch.php?pkg=haskell-unix-time&arch=hurd-i386&ver=0.1.2-3%2Bb1&stamp=1371051034