"make installcheck" and some related cases, when invoked from the toplevel
directory, start out by doing "make all" in src/test/regress.  Since that's
one make recursion level down, the submake-generated-headers target will
do nothing, causing us to fail to create/update generated headers before
building pg_regress.  This is, I believe, a new failure mode induced by
commit 3b8f6e75, so let's fix it.  To do so, we have to invoke
submake-generated-headers at the top level.

Discussion: https://postgr.es/m/0401efec-68f1-679d-3ea3-21d4e8dd11af@gmail.com