Fix scons option "use-system-sm" does not work correctly

1. XP_UNIX, ... should be defined for both of use-system-sm and not (src/third_party/SConscript)
2. scripting engine should link to shim_spidermonkey, not libjs.a (src/mongo/SConscript)

error messages:

1. XP_UNIX, ... should be defined for both of use-system-sm and not (src/third_party/SConscript)

        $ scons --use-system-sm --extralib=js
          :
        ar rc build/darwin/cpppath__opt_local_include_js/extralib_js/use-system-sm/m
        ongo/libmongodandmongos.a build/darwin/cpppath__opt_local_include_js/extrali
        b_js/use-system-sm/mongo/db/connection_factory.o build/darwin/cpppath__opt_l
        ocal_include_js/extralib_js/use-system-sm/mongo/util/net/message_server_port
        .o
        in file included from src/third_party/js-1.7/jspubtd.h:45,
                         from src/third_party/js-1.7/jsapi.h:47,
                         from src/mongo/scripting/engine_spidermonkey.h:32,
                         from src/mongo/scripting/engine_spidermonkey.cpp:20:
        src/third_party/js-1.7/jstypes.h:248:6: error: #error "must define one of xp
        _beos, xp_os2, xp_win or xp_unix"
        src/third_party/js-1.7/jstypes.h:264:2: error: #error no suitable type for j
        sint8/jsuint8
        src/third_party/js-1.7/jstypes.h:277:2: error: #error no suitable type for j
        sint16/jsuint16
          :
        scons: *** [build/darwin/cpppath__opt_local_include_js/extralib_js/use-syste
        m-sm/mongo/scripting/engine_spidermonkey.o] error 1
        scons: building terminated because of errors.

2. scripting engine should link to shim_spidermonkey, not libjs.a (src/mongo/SConscript)

        $ scons --use-system-sm --extralib=js
          :
        scons: *** [build/darwin/cpppath__opt_local_include_js/extralib_js/use-syste
        m-sm/mongo/mongod] Implicit dependency `src/third_party/js-1.7/libjs.a' not 
        found, needed by target `build/darwin/cpppath__opt_local_include_js/extralib
        _js/use-system-sm/mongo/mongod'.
        scons: building terminated because of errors.