[Halld-offline] Issue with Devtoolset-GCC and ROOT 6

[Nathan Sparks]

Building ROOT 6 (cling) with GCC from Devtoolset is not officially supported because of an incompatibility: 
https://sft.its.cern.ch/jira/browse/ROOT-6207 

The recommended build method, with cmake, produces defective products. 
If you try to join strings (with cling), for example, there is a missing symbol error. 

A workaround is to use the old build method instead: 
./configure; make 

Doing so results in cling using the builtin (compatible) string_view.

This workaround has been tested using devtoolset-3 (gcc-4.9.2) on CentOS 6.

The takeaway is:  Use the old build method to compile ROOT if you are using GCC from Devtoolset.

-Nathan