cmake: Enable CMP0042 @rpath usage for MacOSX

Testing:
- should build with homebrew cmake (v3.0.0) without warnings about CMP0042
- should continue to build with older cmake versions on all platforms
- no functional changes on any platform; this is really just quelling a warning with the new cmake

See: http://ci.openmicroscopy.org/view/5.1/job/BIOFORMATS-5.1-merge-cpp/
Note that due to the MacOSX 10.9 node being offline, the job isn't green, but the individual bits of the matrix are green except for 10.9.

http://www.cmake.org/cmake/help/v3.0/policy/CMP0042.html