Improve documentation for build prerequisites for OS X around XCode

[Trott]

• Is a full XCode required?
  • If not, is it recommended?
    • If so, then what is the recommended way to get all the prerequisites?
• If you install XCode, which of the prerequisites do you get automatically? All but Python?

It probably would make sense to split out the OS X prerequisites info from that of other Unix-like operating systems.

[gibfahn]

Let's actually fix this (#13264).