build: Use subdir-objects automake option
This makes automake place the object files in the same subdir as the
source file. For a recursive build system as we have now, there's
no difference, but with a non-recursive build system it means that
the object files don't all end up in the toplevel directory.
diff --git a/configure.ac b/configure.ac
index 7c2a2c5..dd2639f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -21,7 +21,7 @@
AC_USE_SYSTEM_EXTENSIONS
AC_SYS_LARGEFILE
-AM_INIT_AUTOMAKE([1.11 parallel-tests foreign no-dist-gzip dist-xz color-tests])
+AM_INIT_AUTOMAKE([1.11 parallel-tests foreign no-dist-gzip dist-xz color-tests subdir-objects])
AM_SILENT_RULES([yes])