Missing dependency causes breakage on Mac

[hermione521]

Breakage: http://ci.bazel.io/job/rules_rust/BAZEL_VERSION=HEAD,PLATFORM_NAME=darwin-x86_64/259/console

Reason: we now have sandbox on Mac, and the missing dependency cc_wrapper is not in the sandbox so we can't find it.

Suggested fix: add dependency //tools/defaults:crosstool to every rule that need gcc.

[davidzchen]

Fixed by #21

[damienmg]

Not yet fixed, apparently the input is not shiped into the sandbox but I just cannot debug it. --sandbox_debug has no effect.