Fix windows build command quoting; add hidden pref for alternate spawns

Fixes #943. `system()` has issues with quoting of spaces, but may still
be needed due to bugs in the other spawning options. `system()` will be used
if `win_spawn_mode=1`, otherwise leave it at 0.

0 = GLib sync spawn, doesn't capture stderr when there's also stdout (e.g. for make)
1 = system() - currently doesn't quote spaces correctly
2 = custom spawn, hangs for 30s when > 4kb of output.