Merge branch 'feature/FS1654-improve-dune-enable-all-packages'

This branch resolves somes smaller problems with the
dune_enable_all_packages() feature and importantly provides support for
modules with libraries and CMake < 3.1.

* feature/FS1654-improve-dune-enable-all-packages:
  [Release][CMake] Add support for compile options in DuneEnableAllPackages
  [Release][CMake][Bugfix] Use correct test in if() command
  [Release][CMake] Add dune_target_enable_all_packages()
  [Release][CMake][Bugfix] Remove stray unset() command
  [Release][CMake] Convert macros to functions in DuneEnableAllPackages
  [Release][CMake] Improve documentation in DuneEnableAllPackages.cmake

(cherry picked from commit 7cc5bc4b)
Signed-off-by: Steffen Müthing <muething@dune-project.org>