autogen.sh: Fix missing file ar-lib

After autogen.sh --clean, all auto-generated files need to be
re-created. Before this has happened, autoreconf complains
with

  configure.ac: error: required file 'ar-lib' not found

and signals an error on exit.

This change fixes the issue by touching ar-lib to force creation of an
empty file if it does not exist. Because autoreconf is called with the
options --force --install, this file will get replaced afterwards.

autogen.sh

@@ -128,6 +128,11 @@ test -n "$srcdir" || srcdir=.
 # not struggle with old versions of aclocal.
 mkdir -p $srcdir/m4
 
+# create ar-lib if not present to avoid autoreconf throwing an error
+# when the file is missing. As autoreconf is called with --force
+# the file will get updated with the proper contents afterwards.
+touch ar-lib
+
 echo "Generating needed autotools files for $PROJECT by running autoreconf ..."
 autoreconf --force --install --verbose "$srcdir"