test: Only build death tests on platforms that support them

googletest does not support death tests on FreeBSD.  I've submitted a
patch upstream that trivially enables support, but in the meantime we
can't compile death tests for Ceph on FreeBSD.

https://groups.google.com/forum/#!topic/googletestframework/tjY6UjWgNOw

Signed-off-by: Alan Somers <asomers@gmail.com>
Merge pull request #830 from ceph/port/gtest-death-tests

test: Only build death tests on platforms that support them