I see that the kind of check you are talking is recently added by commit 352f6f2d. I think it is better to be consistent in all places. Let's pick one and use that if possible.
Currently there are two constructs to test the same logic, which is not great. I think that using _MSC_VER makes it seem as MSVC exclusive code, when MinGW should also be considered.
In the longterm aligning Postgres with MS product obsolescence will make these tests unneeded, but I can propose a patch for making the test consistent in all cases, on a different thread since this has little to do with $SUBJECT.
