[CMake][Release] Fix dune_enable_all_packages()-related problems

This branch fixes a number of problems related to
dune_enable_all_packages():

- The feature requires at least CMake 2.8.12, which is more recent than
  the baseline requirement of the core modules (2.8.12). We now check
  for that when executing the relevant functions and also remind the
  user to set the correct version in cmake_minimum_required().
- Updated the documentation to also state the version requirement.
- The duneproject script now also warns the user about the more
  stringent CMake version requirements when using the feature and sets
  the correct minimum required CMake version in the new module's main
  CMakeLists.txt file.
- There was a bug in duneproject that caused it to always enable
  dune_enable_all_packages(), ignoring the choice made by the user. That
  bug is fixed in this branch.
- Some minor fixes to duneproject while I was at it.

This fixes FS1678.

* feature/FS1678:
  [dunecontrol] Output choice of external package handling and required CMake version
  [duneproject] Allow specifying ENABLE_ALL as 5th command line argument
  [duneproject] Update CMake minimum version if ENABLE_ALL is true
  [duneproject][Bugfix] Make sure dune_enable_all_packages() can actually be disabled
  [CMake] Improve documentation in DuneEnableAllPackages.cmake
  [CMake][Doc] Add required CMake version to dune_enable_all_packages()
  [CMake] Require CMake 2.8.12 for dune_enable_all_packages()
  [CMake] Fix typo in warning message.
  Add a warning that some functions are 2.8.12

(cherry picked from commit 90411efd)
Signed-off-by: Steffen Müthing <muething@dune-project.org>