build: Add "dist" and "distcheck" targets

* CMakeLists.txt: Include G10AddDistTargets. Set EXTRA_DIST. Add "dist"
and "distcheck" target.
* cmake/modules/G10AddDistTarget.cmake, cmake/modules/g10_dist.sh.in,
cmake/modules/g10_distcheck.sh.in: New.

This adds "dist" and "distcheck" build targets which are similar to the
corresponding automake targets. The tarball is created with git-archive
so that that the targets can only be used in a cloned repo.

• GnuPG-bug-id: T7262