Use --disable-auto-import linker switch in Mingw builds, too.

This is evidently the default on buildfarm member narwhal, but that
is a pretty ancient Mingw version, and there is reason to think that
more recent versions of GNU ld have this feature turned on by default.
Since we are trying to achieve consistency of link behavior across
all Windows toolchains, let's just make sure here.

src/template/win32

+# src/template/win32
+
 # --allow-multiple-definition is required to link pg_dump because it finds
-# pg_toupper() in both libpq and pgport
-LDFLAGS="-Wl,--allow-multiple-definition"
+# pg_toupper() etc. in both libpq and pgport
+# --disable-auto-import is to ensure we get MSVC-like linking behavior
+LDFLAGS="-Wl,--allow-multiple-definition -Wl,--disable-auto-import"