determine correct way to express libxul dependency on SpiderNode on Linux

[mykmelez]

toolkit/library/moz.build says:

# Force libxul to depend on the SpiderNode libs.  This shouldn't be here,
# but I haven't been able to figure out how to get it to happen on Linux
# without this hack.  Ugh.
# TODO: figure out the right way do this.
USE_LIBS += [
    'static:/positron/app/spidernode/.libs/cares',
    'static:/positron/app/spidernode/.libs/http_parser',
    'static:/positron/app/spidernode/.libs/node',
    'static:/positron/app/spidernode/.libs/openssl',
    'static:/positron/app/spidernode/.libs/spidershim',
    'static:/positron/app/spidernode/.libs/uv',
    'static:/positron/app/spidernode/.libs/zlib',
]

So we should do what it says.