configure: Allow to detect git checkout if .git is not a directory

configure: Allow to detect git checkout if .git is not a directory

E.g. if you use openvpn as a git submodule, it can be a
file that points to the parent's git directory.

The changes to Makefile.am are not strictly required, they
still work with that case, but I wanted to keep the usage
consistent.

Change-Id: I9b7a3df012e7606ddb04a944e3fa33247180e8dd
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Acked-by: Yuriy Darnobyt <yura.uddr@gmail.com>
Message-Id: <20240906172112.87148-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29092.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>