mac: Fix gyp tests to pass with Xcode 4.6

Changes:
* cc is now clang, not gcc
* move sdk to 10.7
* make an error message slightly less specific
Review URL: https://codereview.chromium.org/12705005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1594 78cadc50-ecff-11dd-a971-7dbc132099af
improve xcode_emulation.py

+ Improved SDK path lookup: now also supports iPhoneOS and iPhoneSimulator SDKs
+ Adds support for options CLANG_CXX_LANGUAGE_STANDARD, CLANG_CXX_LIBRARY,
  CLANG_WARN_CONSTANT_CONVERSION
+ Adds support for IPHONEOS_DEPLOYMENT_TARGET (iphoneos and iphonesimulator)

BUG=none
Review URL: https://codereview.chromium.org/12725005/

Path from Kal Conley <kcconley@gmail.com>!

git-svn-id: http://gyp.googlecode.com/svn/trunk@1595 78cadc50-ecff-11dd-a971-7dbc132099af