Modules didn't correctly define their dependencies before, which relied
on the frontends implicitly including every module for linking to
succeed.

Also changed every target_link_libraries call to specify visibility of
dependencies to avoid leaking definitions to dependents when not
necessary.

Now cmake relies on two submodules to copy the libraries in the proper folders

This requires bumping up to a minimum of CMake 3.1. The benefit
of using the imported target is that you can switch to the -pthread
compiler flag on request, which may be necessary for some systems if
available.