pgsql: Fix misplacement of submake-generated-headers prerequisites.

Fix misplacement of submake-generated-headers prerequisites.

The sequence "configure; cd src/pl/plpython; make -j" failed due to
trying to compile plpython's .o files before the generated headers
finished building. (This is an important real-world case, since it's
the typical second step when building both plpython2 and plpython3.)
This happens because the submake-generated-headers target is not
placed in a way to make it a prerequisite to compiling the .o files.
Fix that.

Checking other uses of submake-generated-headers, I noted that the one
attached to pg_regress was similarly misplaced; but it's actually not
needed at all for pg_regress.o, rather regress.o, so move it to be a
prerequisite of that.

Back-patch to 9.6 where submake-generated-headers was introduced
(by commit 548af97fc). It's not immediately clear to me why the
previous coding didn't have the same issue; but since we've not
had field reports of plpython make failing, leave it alone in the
older branches.

Pavel Raiskup and Tom Lane

Discussion: <1925924(dot)izSMJEZO3x(at)unused-4-107(dot)brq(dot)redhat(dot)com>

Branch
------
REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/68fb75e103e78478ef45a71e1a25055751e789f2

Modified Files
--------------
src/pl/plpython/Makefile | 4 +++-
src/test/regress/GNUmakefile | 4 +++-
2 files changed, 6 insertions(+), 2 deletions(-)