[Halld-offline] upgrading Xerces-C++ to 3.1.2

[Mark Ito]

On Thursday I plan to switch the default version of the Xerces C++ 
library from 3.1.1 to 3.1.2 here at JLab. We discussed this at the last 
offline meeting:

https://halldweb.jlab.org/wiki/index.php/GlueX_Offline_Meeting,_January_6,_2016#Upgrade_Xerces_C.2B.2B_from_3.1.1_to_3.1.2 
.

As part of the switch both JANA and sim-recon will have to be re-built 
against 3.1.2. The new line-up is reflected in version_1.12.xml, which 
can be found at

   https://halldweb.jlab.org/dist/version_1.12.xml ,

but note that that will not become the default file until Thursday.

Let me know if there are objections.