Fix using sibling-root libuv for Windows builds

HAVE_UV_IMPORT and other config.h macros must not be set unconditionally because no existing libuv release exposes uv_import() and/or uv_export() yet. Windows builds not passing an explicit path to libuv to win32utils/Configure are currently broken because of this, so comment out the offending lines and describe when the aforementioned config.h macros should be set.