stop Xcode tools from overriding deps when CLT is not installed

Some formulas depend on newer versions of some tools than those Apple ships; in those cases, the dependencies are installed keg_only, and the formula needs to see them instead of the system-provided versions. This is achieved by adding dependencies to the path when building a formula.

However, for systems that have no CLT installed (and which are earlier than 10.9), we have to explicitly add the tools from Xcode to the path. This is currently done before adding dependencies. The result is that these tools incorrectly override the newer versions Homebrew provides. For instance, I had this problem with Octave, which depends on a newer version of bison.

This patch fixes the issue by adding dependencies to the path before the Xcode tools are added.