[Halld-offline] hdpm: Mac OS X distribution update

[Nathan Sparks]

Mac OS X distribution users,

I have updated the latest OS X distribution to use Apple LLVM 7.3.0, xerces-c-3.1.3, and the latest jana.
Before updating to the latest (with “hdpm fetch-dist”) you need to:
1. Upgrade OS X to 10.11.4 and the command line tools through the App Store.
2. Upgrade xerces-c through Homebrew.
	- brew update
 	- brew upgrade xerces-c

Of course, you can continue using older distributions without needing to update.
To see a list of the available tarfiles do:
hdpm fetch-dist -l
If you accidentally update to the latest version without having the updated dependencies,
you can go back to an earlier version by specifying the commit corresponding to an earlier distribution tarfile, e.g.
hdpm fetch-dist f959ac6

Best regards,
-Nathan