add subdir-objects to AUTOMAKE_OPTIONS, this builds object files in subdirs with non-recursive make

Makefile.am

@@ -5,7 +5,7 @@
 
 # "foreign" means we don't follow GNU package layout standards
 # 1.7 means we require automake vesion 1.7
-AUTOMAKE_OPTIONS = foreign 1.7
+AUTOMAKE_OPTIONS = foreign 1.7 subdir-objects
 
 noinst_LIBRARIES=
 EXTRA_DIST=