Update bundled gyp for CLT-only OS X

[jacknagel]

Currently it doesn't seem to be possible to compile rust on OS X with only the "Command Line Tools" (CLT) package installed; one must have a full copy of Xcode.

This is a limitation of the bundled copy of gyp, which does not correctly interact with the CLT package. However, upstream gyp has patches that fix this. It would be great if the submodule could be updated.

Thanks!

[alexcrichton]

cc @brson, although I don't mind moving around the gyp clone