linking both at the same time requires more than my 16GB of RAM, so
introduce a .gitmodules field which serializes the two.
in practice, it is just interpreted as an optional dependency. the CI
system won't be taught about it, so this will not cause an unnecessary
serialization there.

Change-Id: I938537ebd3767a1184a5a31c4c026c70a05d499a
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>