Backports from 72X.

- Use own loader to prevent TLS issues.
- Move config to the one which support big plugins. Disable as there are
  no big plugins in 71X.
- Cleanup gcc / llvm toolfiles.
- Advance dcap to latest version.
- Disable a few things (documentation / rpath) in xz. Not really needed
  but since we build root due to dcap in any case.
Merge pull request #893 from ktf/backports

Backports from 72X.
Merge remote branch 'origin/IB/CMSSW_7_1_X/stable' into IB/CMSSW_7_0_X/stable