[dist] Do not include .git link files in dist tarball

Recent versions of Git co-host submodule `GIT_DIR`s in `<root-project>/.git/`
by default, and write their relative path into `<subproject>/.git`,
which is a regular file:

    $ ls -l external/cecil/.git
    -rw-r--r-- 1 foo  bar  42 Jun 16 12:50 external/cecil/.git
    $ cat external/cecil/.git
    gitdir: ../../.git/modules/external/cecil

Relax the expression in `Makefile.am` to also delete these (broken)
"symlinks" when preparing a distribution tarball.