All source files from there are now included in the libgromacs, together
with mdlib. A separate CMakeLists.txt was kept in the subdirectory to
configure the kernels etc. This required some serious hacking with
CMake, because several things were still required to be set in the same
CMakeLists.txt where the library target is created, i.e., in
src/gromacs/CMakeLists.txt. The solution could probably be improved,
but at least it seems to work as it is now.