DOCS: devtoolset-2 dependency for CentOS 6

Add documentation (and/or Redbook items) for devtoolset-2 as a dependency on Centos6 systems.

Further document (in documentation and/or RedBook) that if you are using the command line eclcc compiler to create standalone executables on a system that requires devtoolset-2, you will have to set the CL_PATH explicitly yourself.