contrib: fix install script to be able to run from tarball

The file autogen.sh is not included in tarballs.  Use configure.ac
instead to detect topdir.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

contrib/void/install.sh

@@ -10,7 +10,7 @@ fi
 # Adjust base directory
 if [ -e xbps ]; then
     cd ../..
-elif [ ! -e autogen.sh ]; then
+elif [ ! -e configure.ac ]; then
     echo "*** Please run this script from the Finit base directory."
     exit 1
 fi