Append xcodebuild version to xctool build output directory.

After Xcode 6 support was added several macros were introduced to simplify switching between Xcode 5 and Xcode 6 implementations. These macros cause different runtime issues after switching from Xcode 5 to 6 or back that are hard to debug.
Let's improve build scripts so builds for different versions of xcodebuild are saved in different directories.
Merge pull request #412 from facebook/nekto/improve_build_script

Append xcodebuild version to xctool build output directory.