autogen.sh: touch stamp-h.in after running autoheader

This prevents autoheader from being rerun by make if somebody changes
configure.ac or aclocal.m4 and runs autogen.sh.

Suggested by Christian Franke <Christian.Franke@t-online.de>

autogen.sh

@@ -4,6 +4,7 @@ set -e
 
 autoconf
 autoheader
+echo timestamp > stamp-h.in
 for rmk in conf/*.rmk; do
   ruby genmk.rb < $rmk > `echo $rmk | sed 's/\.rmk$/.mk/'`
 done