Set CMake policy CMP0042

to remove the warning below when running cmake.
In the long run changing the CMake rules to set MACOSX_RPATH
might be the better option.

  CMake Warning (dev):
    Policy CMP0042 is not set: MACOSX_RPATH is enabled by default.  Run "cmake
    --help-policy CMP0042" for policy details.  Use the cmake_policy
    command to set the policy and suppress this warning.
    MACOSX_RPATH is not specified for the following targets:

     libledger

  This warning is for project developers.  Use -Wno-dev to suppress it.
Merge pull request #345 from afh/pull/cmake_warning

Set CMake policy CMP0042