Improve use of object libraries in libgromacs

Improve use of object libraries in libgromacs

Code such as thread-MPI, linear algebra, the selection parser, and
vmddlopen is external to GROMACS and needs different handling for
things like suppressing compiler warnings.  In modern CMake, this is
best done with OBJECT libraries.

This changes prepares for modernizing how compiler flags are handled.

Removed some warning suppressions that were useful only for the
generated groups-scheme kernel files.

Change-Id: I364ad7b3be7f82ba0f583583e502c4d229a21e9a