openconnect: fix --HEAD to report proper version

openconnect ships with a version.sh script to determine the current version,
but it expects to run from within a clean git clone, which it determines in
part by checking for [-d .git]. For it to work properly we need to:

1. symlink to the git clone (so that the [-d .git] test works).
2. clone without --shallow (so that git describe can find a proper tag)
3. copy autogen.sh before modifying it (so that the checkout isn't dirty)

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>