pgoutput evidently needs to be built without -DBUILDING_DLL.  (It seems
like a pretty bad idea that these makefiles need to know exactly where
all the shlibs are in the tree, or maybe what's bad is putting them under
src/backend/.  But right now is not the time to redesign that.)

Also, remove "override CPPFLAGS" in pgoutput's Makefile.  I don't think
that that actually has any bad consequences, but it's certainly useless
in a directory that has no .h files, and it might be contributing to the
failure somehow.

Per buildfarm.