On Darwin, the last argument to GCC is coming up as an empty string.
This is breaking the build of proto_library targets. However, I was not
able to reproduce with the example cpp project[0].
This commit patches the cc_wrapper of Bazel that gets installed on
Darwin to remove the last argument if it's an empty string. This is
not a probem on Linux.
[0]: https://github.com/bazelbuild/examples/tree/master/cpp-tutorial/stage3
