Enable automake's subdif-objects option.
diff --git a/configure.ac b/configure.ac
index aca2a21..34858f6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -8,7 +8,7 @@
 
 AC_GNU_SOURCE
 
-AM_INIT_AUTOMAKE([1.11 gnu nostdinc check-news color-tests silent-rules])
+AM_INIT_AUTOMAKE([1.11 gnu nostdinc check-news color-tests silent-rules subdir-objects])
 AM_SILENT_RULES([yes])
 AM_MAINTAINER_MODE([enable])