[CMSSW_7_0_X] LLVM/Clang, CGAL, GIT, xrootd, data-MagneticField-Interpolation

- Fix GIT PERL5LIB path on SLC6
- Update xrootd version to 3.3.3-rc2.
- Update LLVM/Clang to 3.3 (LLVM r183501, Clang r183502) and enable `-O4` on Linux paltform.
- Add new external CGAL 4.2
- Update data-MagneticField-Interpolation version to V00-05-00 and change the way data package is built.