The MSVC build system already did this, and commit
617dc6d2 used it in a second file.
Back-patch to 9.4, like that commit.

Discussion: https://postgr.es/m/CAA8=A7_1SWc3+3Z=-utQrQFOtrj_DeohRVt7diA2tZozxsyUOQ@mail.gmail.com