Commit
b29e2867 added gnulib's src/assure.h during bootstrap to fix
development builds, but forgot to check that tarballs also build.
With this patch, things get further, although I'm still facing a
failure because the VPATH 'make check' portion of distcheck is
failing to load modules, where an in-tree build succeeded.
* Makefile.am (src_m4_SOURCES): Add assure.h.
Signed-off-by: Eric Blake <eblake@redhat.com>