Move the block for adding the Mac OS X specific properties to after the registration of the library

I just cloned the repo and tried to `cmake` it. I got the following error:

    CMake Error at CMakeLists.txt:12 (set_target_properties):
      set_target_properties Can not find target to add properties to: dumb

Moving this to after the `add_library` statement fixed the problem. This pull request is the realization of that fix.