xrootd-4 once more ...

Brian asked me to re-try building of root with xrootd-4.

With 7_3_X/next + xrootd-4.0.3 the thing compiled out of the box, libNetx and libNetxNG both got built ok. 

Giulio bumped root to 5.34.21 a while back so this might be what fixed the issue. Using xrootd-4.0.3 almost certainly doesn't change anything.

Notes:
- root got linked against system libpng and libjpeg and install of rpm failed afterwards. I added a fix to root.spec. This uses some perlish stuff.
- handling of %online tag in root.spec is inconsistent ... we should ask what they really need these days and get things done right.
- I switched to usage of bulitin GLEW in root. There is no point in having it as external dependency + we do some hacks in root's version to support both x11 and cocoa on osx.

This will also require changes in cmssw, XrdAdapter at least, Brian says things should be ready and can be excavated if needed.