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

Recent versions of Git co-host submodule GIT_DIRs 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.
Merge pull request #1140 from kitsilanosoftware/no-gitlink-in-dist

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