pgsql: Fix pgxs.mk to not try to build generated headers in external bu

Fix pgxs.mk to not try to build generated headers in external builds.

Per Julien Rouhaud and the buildfarm. This is not quite Julien's
patch: there's no need to lobotomize this build rule when building
contrib modules in-tree, so set NO_GENERATED_HEADERS only if PGXS.

In passing, also set NO_TEMP_INSTALL in external builds. This doesn't
seem to be fixing any live bug, because "make check" in an external
build just produces the expected error message without first trying to
make a temp install ... but it's far from obvious why it doesn't, so
this change seems like good future-proofing.

Julien Rouhaud and Tom Lane

Discussion: https://postgr.es/m/CAOBaU_YH=g68opbbMk8is3jNwhoXGa8ckRSre1nx0Obe1C7i-Q@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1a40485af6e43be501500a88b1b9765cc0d69c0b

Modified Files
--------------
src/makefiles/pgxs.mk | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)