Allow using object libraries in conjunction with shared libs.

CMake's object libraries cannot judge whether they will be used
in shared libraries. To allow ease of in core and downstream modules
we resort to compiling position independent code whenever shared DUNE
libraries are requested. This is done by setting CMAKE_POSITION_INDEPENDENT_CODE
to ON. Please not that the can be overriding manually by setting the target
property POSITION_INDEPENDENT_CODE to something else where needed.