Toggle CMake policy CMP0042 on by default

This resolves the project developer warning notice I get from CMake upon generating project files. It is set to prefer the new (v3.0+) CMake policy regarding the use of MACOSX_RPATH. The rationale for this is is ease of use with the resulting built library files under OSX -- it allows me to simply copy the dynamic library files over to my project's dependencies directory path, and be done with it.

This commit does not take into consideration the recent OSX framework bundle feature that has been added, but I do believe that the two *should* work seamlessly together.