configure.ac: Add subdir-objects

Apparently this is required under latest autotools versions.
diff --git a/configure.ac b/configure.ac
index acd153d..c492277 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2,7 +2,7 @@
 	m4_esyscmd([./git-version-gen .tarball-version]),
 	[openbsc@lists.osmocom.org])
 
-AM_INIT_AUTOMAKE([foreign dist-bzip2 no-dist-gzip 1.6])
+AM_INIT_AUTOMAKE([foreign dist-bzip2 no-dist-gzip 1.6 subdir-objects])
 AC_CONFIG_TESTDIR(tests)
 
 dnl kernel style compile messages