contrib: drop os marker files, check Makefile instead

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

contrib/void/install.sh

@@ -8,7 +8,8 @@ if [ "x`id -u`" != "x0" ]; then
 fi
 
 # Adjust base directory
-if [ -e xbps ]; then
+grep -q void Makefile 2>/dev/null
+if [ $? -eq 0 ]; then
     cd ../..
 elif [ ! -e configure.ac ]; then
     echo "*** Please run this script from the Finit base directory."