xcode_emulation: work in the absence of xcodebuild

OS X systems running only the Command Line Tools for Xcode package,
without the full Xcode, don't have a functioning xcodebuild, but this
isn't mandatory for building many gyp projects (e.g. node.js, v8).
This commit handles xcodebuild failures and avoids populating
Xcode-specific CFLAGS/LDFLAGS when xcodebuild can't be run.

This has been tested on both Xcode and CLT-only systems by successfully
building node.js. The behaviour can be simulated on systems with Xcode
by setting the xcode-select path to something nonsensical, e.g.
xcode-select -switch /usr/bin

BUG=https://code.google.com/p/gyp/issues/detail?id=292
Review URL: https://codereview.chromium.org/102733012/

Patch from mistydemeo@gmail.com!



git-svn-id: http://gyp.googlecode.com/svn/trunk@1819 78cadc50-ecff-11dd-a971-7dbc132099af