Determine Xcode status via /Library/Developer.

On Mavericks, and possibly even earlier, /usr/bin/cc and friends are always
installed but just ask you (via a GUI, annoyingly) to install Xcode.  This
makes automated installation difficult.

Empirically, /Library/Developer doesn't seem to exist until the tools really
are installed so we can safely use that to determine whether to install the
Xcode Command Line Tools package.