Build fails when using Xcode command line tools rather than full Xcode

[fu5ha]

Details here: https://github.com/EmbarkStudios/rust-gpu/issues/122

Apparently, according to https://github.com/nodejs/node-gyp/issues/569, the "error"(warning?) can actually just be ignored and continue building as normal. I'm not sure exactly what needs to be changed here to do so.