Replaced our usage of LINK_PRIVATE with that of LINK_INTERFACE_LIBRARIES.

The reasons for that are twofold:

1) LINK_PRIVATE is only available since CMake 2.8.7.
2) The way it was used generated a warning because of CMake policy CMP0023:

   http://www.cmake.org/cmake/help/v2.8.12/cmake.html#policy:CMP0023

Using LINK_INTERFACE_LIBRARIES actually causes another warning - this time
because of CMake policy CMP0022:

   http://www.cmake.org/cmake/help/v2.8.12/cmake.html#policy:CMP0022

I set the policy to OLD, because NEW means subtle changes when compiling
with CMake 2.8.12, and I don't want to research that this close to release.
:-)

I also removed the setting of CMP0003, because it's set by
cmake_minimal_version anyway.