build.pl parses the Makefile to determine the list of sources, etc. Essentially, your extension is built using the unix Makefile. Since you've already built Pg its self from sources you will have used build.pl to do it (right?) and it'll be easy for you to add the contrib module and re-run.
The downside of this approach is that build.pl isn't a complete Makefile parser, and is easily confused by anything but the most trivial Makefile syntax. I had problems when I was putting the crashdump module together because build.pl didn't understand ifndef .
-- Craig Ringer
Ok, I hadn't realized that the VS build system extrapolates information from unix Makefiles..
I briefly tried adding a new contrib module for my extension and re-running the VS build (src\tools\msvc\build.bat), and still get the same error. (about redefinition of errcode). Somehow, it seems to me that there's an inherent incompatibility..
