Add automake subdir-objects option

This addresses autoreconf output such as:

  tools/Makefile.am:37: warning: source file '../backend/sane_strstatus.c' is in a subdirectory,
  tools/Makefile.am:37: but option 'subdir-objects' is disabled

The option was introduced in automake version 1.9 and is slated for
unconditional activation with automake-2.0.

configure.ac

@@ -12,7 +12,7 @@ dnl ***********************************************************************
 
 m4_ifdef([AC_CONFIG_MACRO_DIR], [AC_CONFIG_MACRO_DIR([m4])])
 
-AM_INIT_AUTOMAKE
+AM_INIT_AUTOMAKE([1.9 subdir-objects])
 dnl Turn off feature were automake will automatically run autoreconf.
 dnl This is because we check in generated files into CVS which has
 dnl known timestamp issues.