Fix to support git worktree.

* autogen.sh, Makefile.am, doc/Makefile.am: Use -e for testing .git.

--
Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>

autogen.sh

@@ -214,7 +214,7 @@ if [ "$myhost" = "find-version" ]; then
     esac
 
     beta=no
-    if [ -d .git ]; then
+    if [ -e .git ]; then
       ingit=yes
       tmp=$(git describe --match "${matchstr1}" --long 2>/dev/null)
       if [ -n "$tmp" ]; then