fix-gnulib: Add a call to AC_CONFIG_LIBOBJ_DIR

For some reason, the alloca module uses Autoconf's stock LIBOBJ
machinery and things explode if AC_CONFIG_LIBOBJ_DIR is not set
correctly, because the Gnulib macros sure don't.

Add such a call to the included m4 fragment to fix things up.

The amount of magic and poor documentation around this feature is
astounding.