[Release][CMake][Bugfix] Make dune_library_add_sources() work again

dune_library_add_sources() verifies the library name against the
contents of DUNE_ENABLE_ALL_PACKAGES_MODULE_LIBRARIES. I accidentally
broke this by converting dune_enable_all_packages (which defines the
variable) from a macro to a function, which removed the variable from
global scope.

This patch fixes the problem by explicitly exporting the variable to the
parent scope, making it available to dune_library_add_sources() again.

cmake/modules/DuneEnableAllPackages.cmake

@@ -221,6 +221,14 @@ Update the cmake_minimum_required() call in your main CMakeLists.txt file to get
       # ...and add it to all future targets in the module
       link_libraries(${module_lib})
     endforeach(module_lib ${DUNE_ENABLE_ALL_PACKAGES_MODULE_LIBRARIES})
+
+    # export the DUNE_ENABLE_ALL_PACKAGES_MODULE_LIBRARIES variable to the parent scope
+    # this is required to make dune_library_add_sources() work (see further down)
+    set(
+      DUNE_ENABLE_ALL_PACKAGES_MODULE_LIBRARIES
+      ${DUNE_ENABLE_ALL_PACKAGES_MODULE_LIBRARIES}
+      PARENT_SCOPE
+      )
   endif(DUNE_ENABLE_ALL_PACKAGES_MODULE_LIBRARIES)
 
   if(DUNE_ENABLE_ALL_PACKAGES_VERBOSE)