[WIP] CMake 3.9+: RPATH on OSX does not affect install_name

Created by: ax3l

Add policy CMP0068.

Try to vvoid CMake error on conda-forge:

-- Configuring done
CMake Error at /usr/local/miniconda/conda-bld/adios2_1560084365303/_build_env/share/cmake-3.14/Modules/FindBZip2.cmake:79 (add_library):
  Attempting to use @rpath without CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG being
  set.  This could be because you are using a Mac OS X version less than 10.5
  or because CMake's platform configuration is corrupt.
Call Stack (most recent call first):
  cmake/FindBZip2.cmake:10 (include)
  cmake/DetectOptions.cmake:14 (find_package)
  CMakeLists.txt:123 (include)

Issue:

• https://github.com/conda-forge/staged-recipes/pull/8526 / https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=41164
• Similar solution: https://github.com/opencv/opencv/pull/14146