Cache Xcode version at the module level

XcodeVersion may end up being called many times when building large
projects, so cache the version for performance reason. The xcodebuild
command has also been observed to return incorrect values when called
by multiple processes in parallel, so reducing the number of calls
here reduce the risk of encountering this issue.

BUG=408
R=justincohen@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/170773002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1854 78cadc50-ecff-11dd-a971-7dbc132099af
Fix a local rule reference

BUG=None
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/170853002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1855 78cadc50-ecff-11dd-a971-7dbc132099af