dune_enable_all_packages() doesn't really work if a module contains
libraries (it triggers CMP022 and CMP038). This patch works around that
problem and also adds a number of convenience features to the overall
mechanism. In particular:

- dune_enable_all_packages() now accepts optional lists of additional
  include directories and compile definitions that will be applied to
  all targets in the module. There is a new option APPEND that controls
  the placement of those compiler arguments, analogous to
  dune_register_package_flags().

- Libraries need special handling to work around the problem with the
  two CMake policies listed above. Those get triggered because libraries
  defined in the module ended up linking to themselves, and newer
  versions of CMake really don't like that. We can avoid this problem by
  exploiting the fact that the set of libraries contained in
  link_libraries is evaluated only once, at the point when a target is
  created. So we only have to make sure that libraries inside the module
  are created before they are added to link_libraries(). But we have to
  be careful to make sure that link_libraries() is called before the
  user creates any targets that depend on the module library.
  In order to minimize the risk of user error, I have integrated those
  two tasks (creating the library and adding it to link_libraries())
  into the dune_enable_all_packages() macro. So this macro now accepts a
  list of library names (as the multi argument MODULE_LIBRARIES). These
  are automatically created using dune_add_library() (placing them in
  lib/) in the order that they are listed and added to link_libraries()
  afterwards. Users MUST use this mechanism if they want the library to
  be linked automatically to programs in the module. On the other hand,
  you can still manually create libraries, but then you have to link to
  them manually (e.g. the Alberta libs in dune-grid).

- There is a new macro dune_library_add_sources() that can be used to add
  source files to libraries created with the help of
  dune_enable_all_packages(). That macro can be called repeatedly and in
  any subdirectory of the source tree. Using this facility, I think we
  can mostly get rid of OBJECT libraries. :-)

- CMake doesn't like to create a target without any source files, so the
  patch generates a small stub source for each library that contains two
  functions that are named after the library and return the version of
  the associated dune module.