properly include config.h in source files, not headers

* src/Makefile.am: Add $(DEFAULT_INCLUDES) to a couple of our custom
  rules so we pick up the -I for config.h.

* src/*.[ch]: Include the config.h in the C files, not in the headers.

src/guile-vm.c

@@ -39,6 +39,10 @@
  * whether to permit this exception to apply to your modifications.
  * If you do not wish that, delete this exception notice.  */
 
+#if HAVE_CONFIG_H
+#  include <config.h>
+#endif
+
 #include <libguile.h>
 
 int