all dim-indep objects should be only in one library
Variables from dim-indep objects would exist multiple times. Which copy gets used depends on linker details.
When I used dune-corepy on an old Ubuntu 14.04 installation, different copies of variables would be used in different places in the same program, causing the program to misbehave.
This change moves all dim-indep objects into a single library which should avoid such problems.