[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.

(cherry picked from commit 289d3eb6)
Signed-off-by: Steffen Müthing <muething@dune-project.org>