configure.ac: add subdir-objects option to automake (this avoid lots of warnings with recent versions of automake)

configure.ac

@@ -54,8 +54,8 @@ AC_CANONICAL_SYSTEM
 dnl Automake 1.11 introduced `silent-rules' and `color-tests'.  Use them
 dnl when they're available.
 m4_ifdef([AM_SILENT_RULES],
-  [AM_INIT_AUTOMAKE([1.11 -Wall foreign silent-rules color-tests parallel-tests])],
-  [AM_INIT_AUTOMAKE([1.10 -Wall foreign])])
+  [AM_INIT_AUTOMAKE([1.11 -Wall foreign silent-rules color-tests parallel-tests subdir-objects])],
+  [AM_INIT_AUTOMAKE([1.10 -Wall foreign subdir-objects])])
 
 m4_ifdef([AM_SILENT_RULES],
   [AM_SILENT_RULES(yes)])