Match subsurface_get_conf* in windows/macos.c with linux.c

The signatures for subsurface_get_conf* in windows.c and macos.c
was slightly different from those in linux.c, which broke the
build (at least on Mac).

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>