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

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