Make subdir-objects the default.

Automake 1.14.1 warns that this will become the default in the future
and advises enabling this, I assume to catch conflicts. We already
use this option in the opus library.

configure.ac

@@ -19,7 +19,7 @@ AC_CONFIG_MACRO_DIR([m4])
 AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE
 
-AM_INIT_AUTOMAKE([1.11 foreign no-define dist-zip])
+AM_INIT_AUTOMAKE([1.11 foreign no-define dist-zip subdir-objects])
 AM_MAINTAINER_MODE([enable])
 LT_INIT