feature/issue-278: XCode 5

This pull request adds a `TEMPLATE_DEPTH` variable to the makefile under mac only. It is set to a value of 256 if none is provided.

This does not need to be tested by Jenkins since it doesn't affect Windows.

Bob or Jiqiang, can you verify:
- [x] Stan compiles using XCode 5's g++
- [x] Stan compiles using XCode 5's clang++