conditionally use '-' for template-depth option for g++ 4.2, 4.3, 4.4
only adding template-depth flags for Mac when the versions match XCode 5
updating version to match clang++ version (from -dumpversion)
Merge branch 'develop' into feature/issue-278-Xcode5
Merge pull request #281 from stan-dev/feature/issue-278-Xcode5

Fixes #278.
Syncing make/os_mac