pgsql: Try to fix non-MSVC Windows builds in the wake of logical replic

Try to fix non-MSVC Windows builds in the wake of logical replication.

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0502e854640bd024f349c0df46b7dd6812b8c05c

Modified Files
--------------
src/backend/replication/pgoutput/Makefile | 2 --
src/makefiles/Makefile.cygwin | 4 +++-
src/makefiles/Makefile.win32 | 4 +++-
3 files changed, 6 insertions(+), 4 deletions(-)